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.
Merge pull request #2 from inincaas/OWIN-1.0

Correctly implement OWIN InputStream property and GetRawBody()