Remove scope from the token refresh request as it is redundant

Scope is a valid parameter for the Refresh Token request (Sectiom 6 of
RFC 6749), however it's optional and when ommitted is treated as equal
to the scope originally granted by the resource owner. Since the
indented behavior of this convenience method is to create a token
refresh with the full scope, it's redundant to include.

Related to b5870c0 but slightly
different reason.

Source/OIDAuthState.m

@@ -414,7 +414,7 @@ - (OIDTokenRequest *)tokenRefreshRequestWithAdditionalParameters:
                 redirectURL:nil
                    clientID:_lastAuthorizationResponse.request.clientID
                clientSecret:_lastAuthorizationResponse.request.clientSecret
-                      scope:_lastAuthorizationResponse.request.scope
+                      scope:nil
                refreshToken:_refreshToken
                codeVerifier:nil
        additionalParameters:additionalParameters];