Handle custom headers in post request

Issue

As of now, options such as stream, or timeout are not passed to the call to the docker API.

Cause

These options are circulated through the headers of the http query to the docker API. But recent modifications of the requests library state that adding custom headers to an http query should be handled by the code inheriting from "". See:
https://github.com/psf/requests/blob/786255613bd92f87c9c8f066c4271aab1b9eeaad/requests/adapters.py#L401-L413

Solution

Therefore, I think that a add_headers methods should be added to the UnixHTTPAdapter.

Here is my (particularly ugly) workaround:

        def add_headers(self, request, **kwargs):
            from requests.structures import CaseInsensitiveDict
            new_head = dict(request.headers)
            for key, val in kwargs.items():
                new_head[key] = str(val)
            request.headers = CaseInsensitiveDict(new_head)

        cli = docker.APIClient()
        if hasattr(cli, '_transport_adapter'):
            cli._transport_adapter.add_headers = add_headers

The issue points to UnixHTTPAdapter and APIClient; start by tracing how APIClient POST requests reach the adapter and compare the referenced requests adapters.py behavior. Done means custom headers and options such as stream and timeout reach the Docker API through the request path.