Revert "MINOR: cache: allow caching of OPTIONS request"

Revert "MINOR: cache: allow caching of OPTIONS request"

This reverts commit 1263540fe824c74bc3c668fec3476b880d65b27c.

As discussed in issues #214 and #251, this is not the correct way to
cache CORS responses, since it relies on hacking the cache to cache
the OPTIONS method which is explicitly non-cacheable and for which
we cannot rely on any standard caching semantics (cache headers etc
are not expected there). Let's roll this back for now and keep that
for a more reliable and flexible CORS-specific solution later.