Move awkward bulk-read method out of Headers.

This was used for Vary only. Moving it into the callsite simplifies
both.
Merge pull request #713 from square/jwilson_0418_vary_ii

Move awkward bulk-read method out of Headers.
Change the signature of OkResponseCache.maybeRemove.

The method now removes unconditionally, and it's the caller's responsibility
to decide what should be removed.