Handle missing Host: header

Right now, a request without a `Host:` header (nor `X-Forwarded-Host`) will throw an error every time I access `req.host`.

I fixed that to return an empty string.

This also affects `req.subdomains`.  
`"".split(".")` returns `[ "" ]`.  
However, since `subdomains` slices the initial portion of the host anyway, this will still correctly return `[]`.  (unless `subdomain offset` is `0`)