Fix attempt for static_proxy when the STATIC_URL is extern and generic

This code need reviewing. I am wondering how useful was the replace of `host` and `generic_host` in the previous piece of code. So I guess I maybe misunderstood something, hence reviewing.

In any way, this is now working with those configs:
- `STATIC_URL = "/static/"` (usual development server config)
- `STATIC_URL = "//someamazonurl.s3.amazonaws.com/static/"`
- `STATIC_URL = "https://someamazonurl.s3.amazonaws.com/static/"`