mod_brotli, mod_deflate: Restore the separate handling of 304 Not Modified

responses allowing these modules to properly set or fix-up the response
headers such as Vary or ETag.

This change follows up on r1837056 that disabled that special handling and
thus resulted in a potential violation of RFC7232, 4.1:

   The server generating a 304 response MUST generate any of the following
   header fields that would have been sent in a 200 (OK) response to the
   same request: Cache-Control, Content-Location, Date, ETag, Expires,
   and Vary.)

References:
  https://lists.apache.org/thread.html/f5733ca6743757e8aa8b58a0cd9e27680971551c2a20f5606c66507e@%3Cdev.httpd.apache.org%3E
  https://tools.ietf.org/html/rfc7232#section-4.1

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1843242 13f79535-47bb-0310-9956-ffa450edef68