Fix URIPATH / for BrowserExploitServer

This fixes a bug with request.uri being "" when URIPATH is set to /

It was originally reported here:
https://twitter.com/yadolee/status/469388288550109184

Redmine:
https://dev.metasploit.com/redmine/issues/8804

To verify the patch:

- [x] Use a browser exploit that uses the BrowserExploitServer mixin.
- [x] set URIPATH to /
- [x] Make sure you get a shell

Also verify:

- [x] Use the same or a different browser exploit that uses BrowserExploitServer
- [x] Make sure this time you don't modify URIPATH, so it should generate a random one for you.
- [x] Make sure you get a shell.