Guzzle: Fake a Response using a `MockHandler`

Now this will come in handy for testing:

⚗️ One of the most common integrations developers struggle to test is Guzzle. Fortunately, it has a `MockHandler` you can set up to return predefined responses.

Since it uses the existing `Client` and `Response` objects, there's no need to change your implementation.

— Jason McCreary (@gonedark) December 23, 2019