mod_brotli: Handle new 'no-brotli' internal environment variable that

disables Brotli compression for a particular request.

This mimicks how mod_deflate handles the 'no-gzip' env variable, and
should allow seamless migration for configurations that use it.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1762515 13f79535-47bb-0310-9956-ffa450edef68