[Fixes #1283] Add functional tests for WebAPI-style controllers returnin...

...g HttpContent

@rynowak @yishaigalatzer 
I will add a comment here...`PushStreamContent`'s contract is that the user has to indicate that they are done writing to the stream by closing it...the stream they are closing is actually a wrapper delegating stream created by PushStreamContent...

For reference: http://aspnetwebstack.codeplex.com/SourceControl/latest#src/System.Net.Http.Formatting/PushStreamContent.cs