Multiple host headers #921

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

As of version httpx 0.12.1, sync client send multiple host headers when proxied.

To reproduce

import httpx

proxies = {
    "http": "http://127.0.0.1:8080/"
}

with httpx.Client(proxies=proxies) as client:
    client.get('http://ec2-18-sdl-1.compute.amazonaws.com:8080/items')

Expected behavior

There must be only one host header, as some servers (for example uvicorn) will reject requests otherwise.

Actual behavior

Headers host and Host are set.

Debugging material

The second Host header is set in poolmanager._set_proxy_headers()

Environment

• OS: Windows
• Python version: 3.6.8
• HTTPX version: 0.12.1
• Async environment: No
• HTTP proxy: yes
• Custom certificates: No