Add head property to FileDownloadDelegate's Progress/Response struct (#811)

I needed a way to use a `FileDownloadDelegate` task to fish out the
recommended file name.
```swift
let response = try await downloadTask.get()

// access content-disposition
response.head.headers.first(name: "Content-Disposition")
```

The `head` property is an explicitly unwrapped optional because there is
no "default value" to set it to, and it won't be accessed by the user
until it's already been set anyway. This is a little inelegant, so I
could change it to something like below where I fill in bogus init data,
but that seems worse for some reason.
```swift
public struct Progress: Sendable {
    public var totalBytes: Int?
    public var receivedBytes: Int
    public var head: HTTPResponseHead
}

private var progress = Progress(
    totalBytes: nil,
    receivedBytes: 0,
    head: .init(
        version: .init(major: 0, minor: 0),
        status: .badRequest
    )
)
```

Author: gregcotten

Sources/AsyncHTTPClient/FileDownloadDelegate.swift

@@ -19,13 +19,34 @@ import NIOPosix
 /// Handles a streaming download to a given file path, allowing headers and progress to be reported.
 public final class FileDownloadDelegate: HTTPClientResponseDelegate {
     /// The response type for this delegate: the total count of bytes as reported by the response
-    /// "Content-Length" header (if available) and the count of bytes downloaded.
+    /// "Content-Length" header (if available), the count of bytes downloaded, and the
+    /// response head.
     public struct Progress: Sendable {
         public var totalBytes: Int?
         public var receivedBytes: Int
+
+        public var head: HTTPResponseHead {
+            get {
+                assert(self._head != nil)
+                return self._head!
+            }
+            set {
+                self._head = newValue
+            }
+        }
+
+        fileprivate var _head: HTTPResponseHead? = nil
+
+        internal init(totalBytes: Int? = nil, receivedBytes: Int) {
+            self.totalBytes = totalBytes
+            self.receivedBytes = receivedBytes
+        }
     }
 
-    private var progress = Progress(totalBytes: nil, receivedBytes: 0)
+    private var progress = Progress(
+        totalBytes: nil,
+        receivedBytes: 0
+    )
 
     public typealias Response = Progress
 
@@ -133,6 +154,8 @@ public final class FileDownloadDelegate: HTTPClientResponseDelegate {
         task: HTTPClient.Task<Response>,
         _ head: HTTPResponseHead
     ) -> EventLoopFuture<Void> {
+        self.progress._head = head
+
         self.reportHead?(task, head)
 
         if let totalBytesString = head.headers.first(name: "Content-Length"),

Tests/AsyncHTTPClientTests/HTTPClientTests.swift

@@ -729,51 +729,57 @@ final class HTTPClientTests: XCTestCaseHTTPClientTestsBaseClass {
         var request = try Request(url: self.defaultHTTPBinURLPrefix + "events/10/content-length")
         request.headers.add(name: "Accept", value: "text/event-stream")
 
-        let progress =
-            try TemporaryFileHelpers.withTemporaryFilePath { path -> FileDownloadDelegate.Progress in
+        let response =
+            try TemporaryFileHelpers.withTemporaryFilePath { path -> FileDownloadDelegate.Response in
                 let delegate = try FileDownloadDelegate(path: path)
 
-                let progress = try self.defaultClient.execute(
+                let response = try self.defaultClient.execute(
                     request: request,
                     delegate: delegate
                 )
                 .wait()
 
                 try XCTAssertEqual(50, TemporaryFileHelpers.fileSize(path: path))
 
-                return progress
+                return response
             }
 
-        XCTAssertEqual(50, progress.totalBytes)
-        XCTAssertEqual(50, progress.receivedBytes)
+        XCTAssertEqual(.ok, response.head.status)
+        XCTAssertEqual("50", response.head.headers.first(name: "content-length"))
+
+        XCTAssertEqual(50, response.totalBytes)
+        XCTAssertEqual(50, response.receivedBytes)
     }
 
     func testFileDownloadError() throws {
         var request = try Request(url: self.defaultHTTPBinURLPrefix + "not-found")
         request.headers.add(name: "Accept", value: "text/event-stream")
 
-        let progress =
-            try TemporaryFileHelpers.withTemporaryFilePath { path -> FileDownloadDelegate.Progress in
+        let response =
+            try TemporaryFileHelpers.withTemporaryFilePath { path -> FileDownloadDelegate.Response in
                 let delegate = try FileDownloadDelegate(
                     path: path,
                     reportHead: {
                         XCTAssertEqual($0.status, .notFound)
                     }
                 )
 
-                let progress = try self.defaultClient.execute(
+                let response = try self.defaultClient.execute(
                     request: request,
                     delegate: delegate
                 )
                 .wait()
 
                 XCTAssertFalse(TemporaryFileHelpers.fileExists(path: path))
 
-                return progress
+                return response
             }
 
-        XCTAssertEqual(nil, progress.totalBytes)
-        XCTAssertEqual(0, progress.receivedBytes)
+        XCTAssertEqual(.notFound, response.head.status)
+        XCTAssertFalse(response.head.headers.contains(name: "content-length"))
+
+        XCTAssertEqual(nil, response.totalBytes)
+        XCTAssertEqual(0, response.receivedBytes)
     }
 
     func testFileDownloadCustomError() throws {
@@ -3910,23 +3916,27 @@ final class HTTPClientTests: XCTestCaseHTTPClientTestsBaseClass {
         var request = try Request(url: self.defaultHTTPBinURLPrefix + "chunked")
         request.headers.add(name: "Accept", value: "text/event-stream")
 
-        let progress =
-            try TemporaryFileHelpers.withTemporaryFilePath { path -> FileDownloadDelegate.Progress in
+        let response =
+            try TemporaryFileHelpers.withTemporaryFilePath { path -> FileDownloadDelegate.Response in
                 let delegate = try FileDownloadDelegate(path: path)
 
-                let progress = try self.defaultClient.execute(
+                let response = try self.defaultClient.execute(
                     request: request,
                     delegate: delegate
                 )
                 .wait()
 
                 try XCTAssertEqual(50, TemporaryFileHelpers.fileSize(path: path))
 
-                return progress
+                return response
             }
 
-        XCTAssertEqual(nil, progress.totalBytes)
-        XCTAssertEqual(50, progress.receivedBytes)
+        XCTAssertEqual(.ok, response.head.status)
+        XCTAssertEqual("chunked", response.head.headers.first(name: "transfer-encoding"))
+        XCTAssertFalse(response.head.headers.contains(name: "content-length"))
+
+        XCTAssertEqual(nil, response.totalBytes)
+        XCTAssertEqual(50, response.receivedBytes)
     }
 
     func testCloseWhileBackpressureIsExertedIsFine() throws {