Removed empty parameter from query string in OauthPlugin

v3.3.1 fails when we request the Token to Twitter using OauthPlugin. (v3.3.0 works fine). That's because when Guzzle creates the POST request includes all default parameters  (even the empty ones. oauth_token in this case). Twitter throws an error (unauthorized).

I've changed OauthPlugin:prepareParameters to remove the empty parameters. I've also created a new test unit to check this issue (OauthPluginTest::testCreatesStringToSignFromPostRequestWithNullValues)