e.g.: http+unix://unix.socket/var/run/docker.sock/version

e.g.: http+unix://unix.socket/var/run/docker.sock/version

so we can override scheme from `http+unix` to something else

e.g.:

```
$ REQUESTS_UNIXSOCKET_URL_SCHEME=http://sock.local examples/simple-http.py http://sock.local/var/run/docker.sock/version | jq .
{
  "Version": "1.12.3",
  "ApiVersion": "1.24",
  "GitCommit": "6b644ec",
  "GoVersion": "go1.6.2",
  "Os": "linux",
  "Arch": "amd64",
  "KernelVersion": "4.4.0-63-generic",
  "BuildTime": "Mon, 19 Dec 2016 09:20:48 +1300"
}
```