Fix/web module URIs through reverse proxy

Hi,
This patch fixes accessing to the web interface through a reverse proxy and might fix some prefix bug. I've tested it using nginx frontend.
Here is the nginx configuration extract I use:
```
	location /pyload {
		proxy_set_header Host $host;
		proxy_pass https://localhost:8000/pyload;
		proxy_redirect default;
	}
```
And the pyload.conf extract:
```
webinterface - "Webinterface":
	str template : "Template" = default
	bool activated : "Activated" = True
	str prefix : "Path Prefix" = /pyload
	builtin;threaded;fastcgi;lightweight server : "Server" = threaded
	ip host : "IP" = 127.0.0.1
	bool https : "Use HTTPS" = True
	int port : "Port" = 8000
```
Regards