Hi Robie, this fix is in Apache SVN trunk but isn't in upstream stable 2.4 release branch yet so all Ubuntu versions will be affected. In addition, no such patch exists in Debian source. https://anonscm.debian.org/cgit/pkg-apache/apache2.git/tree/modules/proxy/mod_proxy.h#n350 and https://anonscm.debian.org/cgit/pkg-apache/apache2.git/tree/debian/patches
I think the best bet is to add it as a debian patch for Zesty and coordinate it with the Debian Apache maintainers until it lands in a stable upstream release. I adjusted the line number in the patch so it should apply cleanly against Zesty.