[Mono.Android] Read error response in AndroidClientHandler (#180)

Improve error response handling, using [`ErrorStream`][0] if an
error occurs.

> **Response Handling**  
> `HttpURLConnection` will follow up to five HTTP redirects.
> It will > follow redirects from one origin server to another.
> This implementation doesn't follow redirects from HTTPS to HTTP or
> vice versa.
> If the HTTP response indicates that an error occurred,
> `InputStream` will throw an `IOException`.
> Use `ErrorStream` to read the error response.


[0]: https://developer.android.com/reference/java/net/HttpURLConnection.html#getErrorStream()

src/Mono.Android/Xamarin.Android.Net/AndroidClientHandler.cs

@@ -268,10 +268,10 @@ protected void AssertSelf ()
 					// idea. We'll return the response message with all the information required by the
 					// application to fill in the blanks and provide the requested credentials instead.
 					//
-					// We should return the body of the response too but, alas, the Java client will throw
-					// a, wait for it, FileNotFound exception if we attempt to access the input stream. So
-					// no body, just a dummy. Java FTW!
-					ret.Content = new StringContent ("Unauthorized", Encoding.ASCII);
+					// We return the body of the response too, but the Java client will throw
+					// a FileNotFound exception if we attempt to access the input stream.
+					// Instead we try to read the error stream and return an default message if the error stream isn't readable.
+					ret.Content = GetErrorContent (httpConnection, new StringContent ("Unauthorized", Encoding.ASCII));
 					CopyHeaders (httpConnection, ret);
 
 					if (ret.Headers.WwwAuthenticate != null) {
@@ -285,28 +285,20 @@ protected void AssertSelf ()
 					ret.RequestedAuthentication = RequestedAuthentication;
 					return ret;
 			}
-
+			
 			if (!IsErrorStatusCode (statusCode)) {
 				if (Logger.LogNet)
 					Logger.Log (LogLevel.Info, LOG_APP, $"Reading...");
-				Stream inputStream = new BufferedStream (httpConnection.InputStream);
-				if (decompress_here) {
-					string[] encodings = httpConnection.ContentEncoding?.Split (',');
-					if (encodings != null) {
-						if (encodings.Contains (GZIP_ENCODING, StringComparer.OrdinalIgnoreCase))
-							inputStream = new GZipStream (inputStream, CompressionMode.Decompress);
-						else if (encodings.Contains (DEFLATE_ENCODING, StringComparer.OrdinalIgnoreCase))
-							inputStream = new DeflateStream (inputStream, CompressionMode.Decompress);
-					}
-				}
-				ret.Content = new StreamContent (inputStream);
-			} else {
+				ret.Content = GetContent (httpConnection, httpConnection.InputStream);
+			}
+			else {
 				if (Logger.LogNet)
-					Logger.Log (LogLevel.Info, LOG_APP, $"Status code is {statusCode}, returning empty content");
-				// For 400 >= response code <= 599 the Java client throws the FileNotFound exeption when attempting to read from the connection
-				// Client tests require we return no content here
-				ret.Content = new StringContent (String.Empty, Encoding.ASCII);
+					Logger.Log (LogLevel.Info, LOG_APP, $"Status code is {statusCode}, reading...");
+				// For 400 >= response code <= 599 the Java client throws the FileNotFound exception when attempting to read from the input stream.
+				// Instead we try to read the error stream and return an empty string if the error stream isn't readable.
+				ret.Content = GetErrorContent (httpConnection, new StringContent (String.Empty, Encoding.ASCII));
 			}
+
 			CopyHeaders (httpConnection, ret);
 
 			IEnumerable <string> cookieHeaderValue;
@@ -334,6 +326,32 @@ protected void AssertSelf ()
 			return ret;
 		}
 
+		HttpContent GetErrorContent (HttpURLConnection httpConnection, HttpContent fallbackContent) 
+		{
+			var contentStream = httpConnection.ErrorStream;
+
+			if (contentStream != null) {
+				return GetContent (httpConnection, contentStream);
+			}
+
+			return fallbackContent;
+		}
+
+		HttpContent GetContent (URLConnection httpConnection, Stream contentStream)
+		{
+			Stream inputStream = new BufferedStream (contentStream);
+			if (decompress_here) {
+				string[] encodings = httpConnection.ContentEncoding?.Split (',');
+				if (encodings != null) {
+					if (encodings.Contains (GZIP_ENCODING, StringComparer.OrdinalIgnoreCase))
+						inputStream = new GZipStream (inputStream, CompressionMode.Decompress);
+					else if (encodings.Contains (DEFLATE_ENCODING, StringComparer.OrdinalIgnoreCase))
+						inputStream = new DeflateStream (inputStream, CompressionMode.Decompress);
+				}
+			}
+			return new StreamContent (inputStream);
+		}
+
 		bool HandleRedirect (HttpStatusCode redirectCode, HttpURLConnection httpConnection, RequestRedirectionState redirectState, out bool disposeRet)
 		{
 			if (!AllowAutoRedirect) {