Should probably not copy *all* request headers to the fragment request

Looking at the [varnish implementation](https://github.com/varnishcache/varnish-cache/blob/master/bin/varnishd/cache/cache_esi_deliver.c#L161-L179) there are some headers that should be removed from the fragment request, e.g. if-modified-since or content-length.