Fixed background update with "if".

Fixed background update with "if".

Cloned subrequests should inherit r->content_handler.  This way they will
be able to use the same location configuration as the original request
if there are "if" directives in the configuration.

Without r->content_handler inherited, the following configuration tries
to access a static file in the update request:

    location / {
        set $true 1;
        if ($true) {
            # nothing
        }

        proxy_pass http://backend;
        proxy_cache one;
        proxy_cache_use_stale updating;
        proxy_cache_background_update on;
    }

See http://mailman.nginx.org/pipermail/nginx/2017-February/053019.html for
initial report.