Fix for codeplex-324 - Portable PushStreamContent throws on double dispose

An earlier change here makes the PushStreamContent call dispose on any
stream that's passed in to its SerializeToStreamAsyncMethod (only in
portable library). The comments indiciate that this is necessary because
HttpContent doesn't 'own' the stream.

When the stream is disposed, that makes it unreadable for callers, so any
type of system that tries to buffer PushStreamContent is inherently
broken. HttpContent does this by default in a variety of cases including
simply sending a message with HttpClient.

If a dispose call is really needed, it should be performed by whichever
object 'owns' the stream, and definitely not by the PushStreamContent,
which doesn't own the stream. In all the use cases I could find, the
stream is disposed by HttpContent - additionally, in all of these cases
the provided stream is a MemoryStream, which holds no native resources.

This feature is totally broken right now for portable, if there are cases
where the resource ownership model changes for portable as a result of
this fix, we should discuss those, as most scenarios with this class are
broken right now -- it can hardly get worse.

(cherry picked from commit d5188c8a75b5b26b09ab89bedfd7ee635ae2ff17)