Fix urlencode clientid secret

Ok, so this is the updated pull request for urldecoding client_id/client_secret ad stated in http://tools.ietf.org/html/rfc6749#section-2.3.1 and http://tools.ietf.org/html/rfc6749#section-3.2.1.

Import works, the testsuite passes and I added a test case.