Owin Embedded ResponseStream: import fix 

There was a bug reported in the ResponseStream used by the embedded host: https://katanaproject.codeplex.com/workitem/253
It was fixed here: https://katanaproject.codeplex.com/SourceControl/diff/file/view/25dbcc4d482c98e4315473d13fab532f48bc46fa?fileId=src%2FMicrosoft.Owin.Testing%2FResponseStream.cs&olderChangeSetId=615d2ba5e09f3de539a00935c5619f3fda806907

This PR imports this fix.