We now partition the HTTP disk cache based on the Vary response header.
If a cached response contains a Vary header, we look for each of the
header names in the outgoing HTTP request. The outgoing request must
match every header value in the original request for the cache entry
to be used; otherwise, a new request will be issued, and a separate
cache entry will be created.
Note that we must now defer creating the disk cache file itself until we
have received the response headers. The Vary key is computed from these
headers, and affects the partitioned disk cache file name.
There are further optimizations we can make here. If we have a Vary
mismatch, we could find the best candidate cached response and issue a
conditional HTTP request. The content server may then respond with an
HTTP 304 if the mismatched request headers are actually okay. But for
now, if we have a Vary mismatch, we issue an unconditional request as
a purely correctness-oriented patch.
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