Setters that are necessary for simple GoogleCredential creation are overridden in
order to change the return type. A concrete JsonFactory is set by default, since JSON
parsing is relied upon often in GoogleCredential which makes mocking parse calls problematic.
By default, a standard MockHttpTransport is supplied. For simple tests in which
'refresh' methods are called but the request/response isn't used #newMockHttpTransportWithSampleTokenResponse() provides a minimal implementation. For more
complex tests which check request/response behavior prefer MockTokenServerTransport.