Get response headers from request

[eddieharrison]

I wanted to be able to get the response headers from a request to handle the x-rate-limit-remaining in our app.

I've made it an optional setting before the request and added the ability to retrieve them after the request via a private property.

Added a phpunit test for this going forwards too.

[gregorpraprotnik]

@eddieharrison Any plans to move this forward? I'd also need this feature to retrieve rate limit headers.