Fix the OPTIONS response. It was attempting to send an HTTP response

Fix the OPTIONS response. It was attempting to send an HTTP response
(header) through the filter stack, which just wrapped that response in
another set of headers.

Instead, just set the Allow header and return. The EOS will then flush that
header with the rest of the data through the header filter, and generate the
proper response.

Also, cleaned out the unused header_filter_ctx and the "len" variable from
the header filter, and added some comments here and there.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@88600 13f79535-47bb-0310-9956-ffa450edef68