Add GetContentLength() ext method

Author: mythz

src/ServiceStack.Text/HttpUtils.HttpClient.cs

@@ -1122,6 +1122,9 @@ public static class HttpClientExt
 {
     public static bool MatchesContentType(this HttpResponseMessage res, string matchesContentType) => 
         MimeTypes.MatchesContentType(res.GetHeader(HttpHeaders.ContentType), matchesContentType);
+
+    public static long? GetContentLength(this HttpResponseMessage res) =>
+        res.Content.Headers.ContentLength;
 }
 
 #endif

src/ServiceStack.Text/HttpUtils.WebRequest.cs

@@ -1186,5 +1186,8 @@ public static class HttpClientExt
 {
     public static bool MatchesContentType(this HttpWebResponse res, string matchesContentType) => 
         MimeTypes.MatchesContentType(res.Headers[HttpHeaders.ContentType], matchesContentType);
+
+    public static long? GetContentLength(this HttpWebResponse res) =>
+        res.ContentLength == -1 ? null : res.ContentLength;
 }
 #endif