http: Fix possible empty response with mod_ratelimit for HEAD requests.

Don't eat the EOS in ap_http_header_filter() if it comes in single brigade
with a full response to a HEAD request, otherwise mod_ratelimit will never
flush its pending data.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1854004 13f79535-47bb-0310-9956-ffa450edef68