Upstream: handling of proxy_set_header at http level.

Upstream: handling of proxy_set_header at http level.

When headers are set at the "http" level and not redefined in
a server block, we now preserve conf->headers into the "http"
section configuration to inherit it to all servers.

The same applies to conf->headers_cache, though it may not be effective
if no servers use cache at the "server" level as conf->headers_cache
is only initialized if cache is enabled on a given level.

Similar changes made in fastcgi/scgi/uwsgi to preserve conf->params
and conf->params_cache.