Requests from transport API

[tomchristie]

Related to #1091

Improving our support for instantiating a request instance, from the transport API.

• method may now be either str or bytes.
• url now supports passing a raw URL. (bytes, bytes, Optional[int], bytes).

Working towards being able to do something like...

class CustomTransport:
    def request(self, method, url, headers, stream, timeout):
        request = httpx.Request(method, url, headers, stream)
        ...
        response = httpx.Response(...)
        return response.raw

[johtso]

Is the raw property on the request still waiting to be done? Would be useful for the transport tests that I'm fiddling with. Maybe request should expose a similar property that matches the input to the transport .request()?