Stub implementation of EndGetRequestStream overload in HttpWebRequest.cs

I am not sure if a "do-nothing" implementation like this is acceptable in the Mono code base, but it does solve the problem described in https://bugzilla.xamarin.com/show_bug.cgi?id=12875 (successfully executing simple requests using the Google API .NET client library).
Changed compiler directive according to documentation

EndGetRequestStream with TransportContext parameter was introduced in .NET 3.5. Also added "using" statement for System.Net (namespace for TransportContext).
Merge pull request #834 from OttoG/master

Stub implementation of EndGetRequestStream overload in HttpWebRequest.cs