[5.7] Use ClientInterface proper request() method

[lucasmichot]

\GuzzleHttp\ClientInterface::post method does not exist.
We can use \GuzzleHttp\ClientInterface::request instead

[mvdnbrk]

An instance of GuzzleHttp\Client is passed to the constructor which implements the GuzzleHttp\ClientInterface interface. The client has a post method.

[X-Coder264]

@mvdnbrk Yeah, that is true when a GuzzleHttp\Client is injected, but what if we inject some other object that implements GuzzleHttp\ClientInterface? 😄 That instance probably won't have the post method as the interface doesn't force you to implement it so everything would break. What's the point of having an interface then if it isn't gonna be honored?

[mvdnbrk]

@X-Coder264 you have a point there. I have previously committed a Pull Request with some sort of issue and it got rejected by @taylorotwell. Let Taylor decide 👏