Invalidate cached item if HEAD response status is not NOT_MODIFIED. See
http://tools.ietf.org/html/rfc2616#section-9.4, if the new field values
indicate that the cached entity differs from the current entity, then
the cache MUST treat the cache entry as stale.
Reverted previous change to not cache HEAD requests and fixed
compilation problems after cherry-picking change from master.
Bumped version.