Fix issue with different methods used for signing and requesting access token

In the OAuthV1 code that requests an access token, signatures are generated using the request method (among other parameters).  

Without the fix, when the request method (access_token_method property) is set to 'POST', then the signature is generated using 'POST', but the request is made with 'GET'.  This causes a signature mismatch, and the OAuthV1 provider throws an error.

This quick fix passes the method along when requesting an access token, fixing the mismatch.