Test for callback verification

The oauth_callback parameter is never required except for requests for Temporary Credentials as outlined in Section 2.1 of the RFC.  While in the commit submitted the parameter switches off, it may be a wiser idea to leave the check as True so that backward compatibility with existing workarounds are not broken.

That said, implementations using this library should have tests of their own to check against overall correctness. Oh yes, they are supplied here, too.