Add a maybe state to mindlesslyCacheVary.

 * When mindlesslyCacheVary is set to maybe, we try to check other
 header information in the cache to consider if the resource is
 obsolete or not.
 * This is not compliant with the RFC since we should ensure that the
 Vary header is consistent between requests. This is not the case here.
 * This will only "work" if used as a single browser cache.