REG-TESTS: decompression: check the response header rewriting

REG-TESTS: decompression: check the response header rewriting

Covers two of the fixes in this series: an unrelated "Vary" value that is a
prefix of "accept-encoding" ("Vary: Accept") must survive, a strong ETag
must be weakened since the representation changed, and a content coding
which is only a prefix of a configured algorithm ("Content-Encoding: g")
must not engage decompression.

The case-insensitive comparison of the coding is not covered: vtest always
emits its own lower-cased Content-Encoding next to the one given with -hdr,
so the response carries both and the test would pass either way.