Allow another HTTP client

[ElGigi]

Hi,

Can you allow to set another HTTP Client to the library ?
You implement already PSR-7, and you just have #to implement https://github.com/php-http/httplug for example.

Regards.

[stevenmaguire]

This topic has been discussed at length. #538

[soullivaneuh]

@ElGigi Please see #685