Correctly implement OWIN InputStream property and GetRawBody()

When last upgrading the OWIN-backed request implementation, I forgot to
update the InputStream property to match. While that was being fixed,
the implementation of GetRawBody() was easy enough to add as well.

Strangely enough, this resolved the failing unit test.
(I'm kidding, it's not strange. :astonished: I'm just a moron and should remember that unit tests exist for a reason!)