core, mod_rewrite: introduce the 'redirect-keeps-vary' note to allow proper Vary header insertion when dealing with a RewriteRule in a directory context.
This change is an attempt to fix a long standing problem, brought up while working on PR 58231. Our documentation clearly states the following:
"If a HTTP header is used in a condition this header is added to the Vary header of the response in case the condition evaluates to true for the request."
This is currently not true for RewriteCond/Rules working in a directory context, since when an internal redirect happens all the outstanding response headers get dropped.
There might be a better solution so I am looking forward to hear more opinions and comments. My goal for a delicate change like this one would be to affect the least amount of configurations possible, without triggering unwanted side effects.
If the solution is good for everybody tests will be written in the suite asap.