Improve cachability predicates

To the best of my understanding, rfc2616 says we
    MUST NOT:
     - serve unsafe methods (i.e., not GET or HEAD) from cache
    MAY (unless explicitly prohibited):
     - cache successful (2xx) responses
     - cache responses to any method (including POST, etc)

Before this change, we interpreted these requirements to a more strict
interpretation that:
    - never cached non 200 OK responses (didn't cache 201, 202, etc)
    - served only GETs from cache (not HEADs, despite being allowed to)
    - never cached responses to anything but GET

This change improves cachability by laxing our behaviour to:
    - generally willing to cache any 2xx response
    - willing to serve any safe method from cache (GET or HEAD)
    - still never willing to cache anything but GET (same as before)
Bump version, leave beta