Various fixes to PdoTest

See issue #40
* Removed calls to arrayHasKey in unit tests.
* Added test of refresh token.
* Check authorization_code instead of access_token in testSetAuthorizationCode
* Check client_id instead of client_identifier in testGetClientDetails
* Added interface type hinting where applicable. testCheckUserCredentials uses public setUser/getUser which are not part of any interface and can therefore not be type hinted.