genhash: make url default value /, same as curl/wget

Before:
`127.0.0.1 - - [02/Dec/2013:13:12:53 +0100] "GET (null) HTTP/1.0" 400 172 "-" "-"`

After:
`127.0.0.1 - - [02/Dec/2013:16:16:30 +0100] "GET / HTTP/1.0" 200 151 "-" "GenHash (Linux powered)"`
`127.0.0.1 - - [02/Dec/2013:16:19:49 +0100] "GET /pietje HTTP/1.0" 200 151 "-" "GenHash (Linux powered)"`

Wget/Curl example:
`127.0.0.1 - - [02/Dec/2013:16:20:25 +0100] "GET / HTTP/1.1" 200 151 "-" "Wget/1.13.4 (linux-gnu)"`
`127.0.0.1 - - [02/Dec/2013:16:20:55 +0100] "GET / HTTP/1.1" 200 151 "-" "curl/7.26.0"`