Update the default headers with request_kwargs passed to (Async)HTTPProvider

[souliane]

What feature should we add?

Hi,

This is nice: 86827d6
because it allows to build the header without having an instantiated object, and then pass them to the constructor.
(edit: I am actually wondering: why decorate get_request_headers with @combomethod and not @classmethod?!)

But it would be IMHO more straightforward if what is passed in request_kwargs["headers"] was used to extend the result of HTTPProvider.get_request_headers.

I am willing to open a PR, if you agree with this idea.