Add a method for user to delete one of headers from a request

[NewUserHa]

Currently, users can't make a header not to send out in httpx, in practice, like refer: origin: X-Requested-With: is not needed for some requests.

it should allow:

import httpx
s = httpx.AsyncClient(headers={"refer": "example.com", "origin": "example.com"})
r = await s.get("http://example.com", headers={"origin": None})

to archive the same behavior of requests (handle with merge_setting() from prepare_request() of Session() in seesion.py)

Perhaps should make normalize_header_value() of Headers() in '_models.py' to support None to fix
since _merge_headers() of build_request instance both 'session.headers' and 'headers' argument to Headers() beforeupdate()?

(also here should avoid re-instance the self.headers multiple times since it already is Headers()?)

[scottzach1]

Related to #152

[injust]

For my use case, I need to remove the Accept header that is automatically added in the BaseClient.headers setter.

Right now, I'm doing this with:

httpx_client = httpx.AsyncClient()
del httpx_client.headers["Accept"]

Instead, it would be nice if this were possible:

httpx_client = httpx.AsyncClient(headers={"Accept": None})