httpx uses proxy unexpectedly on windows

[amchii]

Checklist

• The bug is reproducible against the latest release and/or master.
• There are no similar issues or pull requests to fix it yet.

Describe the bug

I set a proxy globally on 127.0.0.1:10909 but except for localhost/127.0.0.1(Windows Setting->Network->Proxy ), then I found httpx.get('http://127.0.0.1:8000') will use the proxy(unexpected), but chrome and requests.get('http://127.0.0.1:8000') accessed it directly(expected).

To reproduce

1. Set a proxy globally but except for localhost\127.0.0.1,
2. Exec httpx.get('http://127.0.0.1:8000') .

Expected behavior

httpx.get accesses http://127.0.0.1:8000 without proxy.

Actual behavior

httpx.get accesses http://127.0.0.1:8000 with proxy.

Debugging material

Environment

• OS: Windows 10
• Python version: 3.7.6
• HTTPX version: 0.17.0
• Async environment: no
• HTTP proxy: no
• Custom certificates: no

Additional context