Caching doesn't account for block differences

[jgarber]

The cache block should not reuse the cached result when tag bodies differ. What's inside <r:filckr:photos> may vary from one use to the next, but currently, the result will be the same because the cache doesn't know better. Failing spec at jgarber/flickr_tags@3ab828c

I don't know how to incorporate tag.block into the cache key. The object ID will be different even if the block text is the same, and there's no way to inspect that text by that point.

Maybe we should get rid of caching in light of my performance boost (jgarber/flickr_tags@5fa0edb)?