Check also err_headers_out for an already set Content-Encoding:

gzip header. This prevents gzip compressed content from a CGI
script from being compressed once more.

PR: 17797
Reviewed by: Ian Holsman, Jeff Trawick


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@99198 13f79535-47bb-0310-9956-ffa450edef68