Generalise the content encoding detection and protocol:

Generalise the content encoding detection and protocol:
so it aslo works in inflate out filter, as suggested by rpluem.

NOTE: this fails with some generators (cgi, asis) due to a deeper bug:
content-encoding is set later than mod_deflate sees it.  This has always
been the case, and could use a separate fix if anyone wants inflate_out
other than in a proxy situation.  And it's not mod_deflate's problem.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@562507 13f79535-47bb-0310-9956-ffa450edef68