Fix environment proxies

[karpetrosyan]

Now the httpx parses NO_PROXY: http://google.com to {'all://*http://google.com': None} but expected {'http://google.com': None}

TODO

• Write a failing test
• Make the test pass

[karpetrosyan]

Also, our documentation is now incorrect; according to https://www.python-httpx.org/environment_variables/#no_proxy, we can use scheme filtering with the NO_PROXY environment variable, which we cannot.

[karpetrosyan]

Docs

export HTTP_PROXY=http://my-external-proxy.com:1234
export NO_PROXY=http://127.0.0.1,python-httpx.org

# As in the previous example, this request will be sent through the proxy
python -c "import httpx; httpx.get('http://example.com')"

# These requests will be sent directly, bypassing the proxy
python -c "import httpx; httpx.get('http://127.0.0.1:5000/my-api')"  <- # This is not correct!
python -c "import httpx; httpx.get('https://www.python-httpx.org')"

[tomchristie]

Excellent, yep. Apologies for the delay.

Let's get a CHANGELOG entry and pull this in. 👍

(Maybe this?... "Handle NO_PROXY envvar cases when a fully qualified URL is supplied as the value.")

[karpetrosyan]

Done!