Fix 'Content-Encoding: gzip' missing if the first brigade passed to

deflate_out_buffer contained zero data bytes but no EOS bucket.

Don't compress if the added headers and checksums are larger than
the data to compress (and we know the size of the data in advance).


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1170330 13f79535-47bb-0310-9956-ffa450edef68