Add keyword parameters to authenticators

Username can be set by args[0], authcid, username, or user.
Secret can be set by args[1], secret, or password.

Since all of the existing authenticators have the same API, it is
sufficient to update `check_args` in the base class.

lib/net/smtp/auth_cram_md5.rb

@@ -9,7 +9,12 @@ class Net::SMTP
   class AuthCramMD5 < Net::SMTP::Authenticator
     auth_type :cram_md5
 
-    def auth(user, secret)
+    def auth(user_arg = nil, secret_arg = nil,
+             authcid: nil, username: nil, user: nil,
+             secret: nil, password: nil,
+             **)
+      user   = req_param authcid, username, user, user_arg, "username (authcid)"
+      secret = req_param password, secret, secret_arg,      "secret (password)"
       challenge = continue('AUTH CRAM-MD5')
       crammed = cram_md5_response(secret, challenge.unpack1('m'))
       finish(base64_encode("#{user} #{crammed}"))

lib/net/smtp/auth_login.rb

@@ -2,7 +2,12 @@ class Net::SMTP
   class AuthLogin < Net::SMTP::Authenticator
     auth_type :login
 
-    def auth(user, secret)
+    def auth(user_arg = nil, secret_arg = nil,
+             authcid: nil, username: nil, user: nil,
+             secret: nil, password: nil,
+             **)
+      user   = req_param authcid, username, user, user_arg, "username (authcid)"
+      secret = req_param password, secret, secret_arg,      "secret (password)"
       continue('AUTH LOGIN')
       continue(base64_encode(user))
       finish(base64_encode(secret))

lib/net/smtp/auth_plain.rb

@@ -2,7 +2,12 @@ class Net::SMTP
   class AuthPlain < Net::SMTP::Authenticator
     auth_type :plain
 
-    def auth(user, secret)
+    def auth(user_arg = nil, secret_arg = nil,
+             authcid: nil, username: nil, user: nil,
+             secret: nil, password: nil,
+             **)
+      user   = req_param authcid, username, user, user_arg, "username (authcid)"
+      secret = req_param password, secret, secret_arg,      "secret (password)"
       finish('AUTH PLAIN ' + base64_encode("\0#{user}\0#{secret}"))
     end
   end

lib/net/smtp/authenticator.rb

@@ -15,11 +15,14 @@ def self.auth_class(type)
         Authenticator.auth_classes[type]
       end
 
-      def self.check_args(user_arg = nil, secret_arg = nil, *, **)
-        unless user_arg
+      def self.check_args(user_arg = nil, secret_arg = nil, *,
+                          authcid: nil, username: nil, user: nil,
+                          secret: nil, password: nil,
+                          **)
+        unless authcid || username || user || user_arg
           raise ArgumentError, 'SMTP-AUTH requested but missing user name'
         end
-        unless secret_arg
+        unless password || secret || secret_arg
           raise ArgumentError, 'SMTP-AUTH requested but missing secret phrase'
         end
       end
@@ -52,6 +55,12 @@ def base64_encode(str)
         # expects "str" may not become too long
         [str].pack('m0')
       end
+
+      def req_param(*args, name)
+        args.compact.first or
+          raise ArgumentError, "SMTP-AUTH requested but missing #{name}"
+      end
+
     end
   end
 end

test/net/smtp/test_smtp.rb

@@ -455,6 +455,15 @@ def test_start_auth_plain
       port = fake_server_start(auth: 'plain')
       Net::SMTP.start('localhost', port, user: 'account', password: 'password', authtype: :plain){}
 
+      port = fake_server_start(auth: 'plain')
+      Net::SMTP.start('localhost', port, authtype: "PLAIN",
+                      auth: {username: 'account', password: 'password'}){}
+
+      port = fake_server_start(auth: 'plain')
+      Net::SMTP.start('localhost', port, auth: {username: 'account',
+                                                password: 'password',
+                                                type: :plain}){}
+
       port = fake_server_start(auth: 'plain')
       assert_raise Net::SMTPAuthenticationError do
         Net::SMTP.start('localhost', port, user: 'account', password: 'invalid', authtype: :plain){}