Environment var HTTP_PROXY is not respected when HTTPTransport is passed as argument

[tscheburaschka]

We are using httpx as the basic HTTP-client in a library of custom API connectors.
We would like to configure the number of retries and the only way we found was to pass a httpx.HTTPTransport object to the instantiation of httpx.Client. However, this unexpectedly stopped the client from respecting the environment variable HTTP_PROXY.
If this is intended behavior, it would benefit from explicit documentation.
However, I would expect the env vars still to work, even when a transport is passed.

nb: AFAIK, the critical lines are here https://github.com/encode/httpx/blob/master/httpx/_client.py#L669

[magicbuffer]

I am also affected by this. When using a custom transport, loading proxy configuration from environ is disabled in clients like AsyncClient:

allow_env_proxies = trust_env and app is None and transport is None
proxy_map = self._get_proxy_map(proxies or proxy, allow_env_proxies)

A dummy example to reproduce this (it assumes the only internet access is via a proxy, passed via HTTPS_PROXY):

import httpx
class CustomTransport(httpx.BaseTransport):
    def __init__(self, **kwargs):
        self._wrapper = httpx.HTTPTransport(**kwargs)
    def handle_request(self, request):
        return self._wrapper.handle_request(request)
# OK
with httpx.Client() as c:
    print(c.get('https://google.com'))
# NOK
with httpx.Client(transport=CustomTransport()) as c:
    print(c.get('https://google.com'))

I would love to hear your views on this, is this a design choice? A side effect maybe? Is there any quick fix to bypass this? Should it be addressed in httpx somehow? Happy to send a PR if needed.

Thanks for reading this and for all the work on httpx 💯