With this, if see an URI like:

/_rels/.rels?ContentType=application/vnd.openxmlformats-package.relationships+xml

Then it is properly detected that it's the .rels stream of the _rels
storage, and UriBindingHelper will serve that stream (when looked up by
name later) to libxmlsec.

Change-Id: Iac62cb170c0aa8bb92c40311fb7b248e96c25dde