[Mono.Android+AndroidClientHandler] Use space within User-Agent (#2071)

Fixes: #1931

The HTTP `User-Agent` header must use the space symbol (` `) as a
separator instead of comma (`,`), as per [RFC 7231][0],
which states that [`User-Agent`][1] is:

	User-Agent = product *( RWS ( product / comment ) )

[`RWS` is defined in RFC 7230][2]:

	RWS            = 1*( SP / HTAB )
	               ; required whitespace

([`SP` and `HTAB` in turn come from RFC 5234 Appendix 5.1][3].)

Update `AndroidClientHandler.AddHeaders()` so that space (` `) is
used to separate `User-Agent` values, not comma (`,`).

[0]: https://tools.ietf.org/html/rfc7231
[1]: https://tools.ietf.org/html/rfc7231#page-46
[2]: https://tools.ietf.org/html/rfc7230#section-3.2.3
[3]: https://tools.ietf.org/html/rfc5234#appendix-B.1

Author: rubo

src/Mono.Android/Test/Xamarin.Android.Net/HttpClientIntegrationTests.cs

@@ -468,7 +468,7 @@ public void Send_Complete_CustomHeaders_SpecialSeparators ()
 					var request = l.Request;
 
 					try {
-						Assert.AreEqual ("MLK,Android,Phone,1.1.9", request.UserAgent, "#1");
+						Assert.AreEqual ("MLK Android Phone 1.1.9", request.UserAgent, "#1");
 						failed = false;
 					} catch (Exception ex) {
 						failed = true;

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

@@ -71,6 +71,10 @@ sealed class RequestRedirectionState
 		const string DEFLATE_ENCODING = "deflate";
 		const string IDENTITY_ENCODING = "identity";
 
+		static readonly IDictionary<string, string> headerSeparators = new Dictionary<string, string> {
+			["User-Agent"] = " ",
+		};
+
 		static readonly HashSet <string> known_content_headers = new HashSet <string> (StringComparer.OrdinalIgnoreCase) {
 			"Allow",
 			"Content-Disposition",
@@ -868,13 +872,15 @@ void HandlePreAuthentication (HttpURLConnection httpConnection)
 			httpConnection.SetRequestProperty (data.UseProxyAuthentication ? "Proxy-Authorization" : "Authorization", authorization.Message);
 		}
 
+		static string GetHeaderSeparator (string name) => headerSeparators.TryGetValue (name, out var value) ? value : ",";
+
 		void AddHeaders (HttpURLConnection conn, HttpHeaders headers)
 		{
 			if (headers == null)
 				return;
 
 			foreach (KeyValuePair<string, IEnumerable<string>> header in headers) {
-				conn.SetRequestProperty (header.Key, header.Value != null ? String.Join (",", header.Value) : String.Empty);
+				conn.SetRequestProperty (header.Key, header.Value != null ? String.Join (GetHeaderSeparator (header.Key), header.Value) : String.Empty);
 			}
 		}