If a server varies on the accept-encoding header and it sends a response
with an encoding we do not know (see parse_encoding_value function), we
will not store it. This will prevent unexpected errors caused by
cache collisions that could happen in accept_encoding_hash_cmp.
This patch fixes GitHub Issue #988. Commit ce9e7b2521
was not sufficient, because it fell back to a hash comparison if the bitmap
of known encodings was not acceptable instead of directly returning the the
cached response is not compatible.
This patch also extends the reg-test to test the hash collision that was
mentioned in #988.
Vary handling is 2.4, no backport needed.
