HttpClient instantiation and drop

Breaking Changes

• HttpClient::new() now returns a Result<HttpClient, Error> since creating a client is fallible. Previously this method would panic if instantiation failed, which was not a very API.
• The Default implementation for HttpClient has been removed, since instantiating it is fallible.

Fixed

• Dropping an HttpClient previously would cause any active transfers created by the client to return EOF after emptying the response body buffer. Now the curl multi handle will be kept alive until all transfers are completed or cancelled. In addition, reading from the response body will return a ConnectionAborted error if for some reason the transfer was stopped without finishing. (#65)

Changed

• Use the latest GitHub Actions beta features for CI. (#56)