fix #2311 null GetResponseStream() on HttpWebException

[Mpdreamz]

Confirmed we do not handle the case where response.GetResponseStream() returns null in our WebException handling.

As per: http://msdn.microsoft.com/en-us/library/system.net.httpwebresponse.getresponsestream.aspx

You must call either the Stream.Close or the HttpWebResponse.Close method to close the stream and release the connection for reuse. It is not necessary to call both Stream.Close and HttpWebResponse.Close, but doing so does not cause an error. Failure to close the stream will cause your application to run out of connections.

We rely on disposing the stream only but in this case we need to forcefully call dispose on response instead.