native-fetcher: fix host header handling when no host header is given

When the host header is not explicitly set in the request headers that
are passed in, the native fetcher needs to derive it from the url.

Fixes https://github.com/pagespeed/ngx_pagespeed/issues/347
Merge pull request #349 from pagespeed/oschaaf-native-fetcher-host-handling

 native-fetcher: host header handling when no host header is given