Add `headers` to the object and pass it to childs

[kajbaf]

It would be useful to pass base headers to the URL object and cache it.

Then, any child URLs can inherit this top-level header values.

Any call to HTTP verbs can amend or override cached headers.

root = Path(base_url, headers={"accept": "application/json"}

# get data as json
root.get()

# post data as json
content = root / "content"
content.post(params={"a": "b"}, headers={"content-type": "application/json"})

I'll try to create a patch.