native-fetcher: host handling

Properly overrides the host header using the host from the passed in url, like the Serf fetcher does.

This should fix https://github.com/pagespeed/ngx_pagespeed/issues/347

With this pull however, the system test starts failing earlier when executed using the native fetcher:
```
     check_not fgrep -q .pagespeed. /tmp/mod_pagespeed_test.oschaaf/fetched_directory/output
     failure at line 363
in 'HonorNoTransform cache-control: no-transform'
```

I am fairly sure this proposed change is an improvement. The new failure seems to have a different cause, I will file a new issue for that.