Correct issue with Google refreshToken() method

[iBotPeaches]

Fixes: #683

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When issuing a refreshToken call with Google - you don't get a new refresh token. You are expected to keep the original one. At the moment Socialite makes an assumption that you will - thus crashes with:

Laravel\Socialite\Two\Token::__construct(): Argument 2 ($refreshToken) must be of type string, null given, called in vendor/laravel/socialite/src/Two/AbstractProvider.php on line 353.

Overrode refreshToken with specific implementation on Google, then added a test.