I think Array#to_h got added in ruby 2.0, but CI passed and we have ruby 1.9.3 in travis.  It looks like this is passing because QueryApi#fetch_multi also checks for `IdentityCache.should_cache?`.  Please use `Hash[keys.zip(results)]` instead, unless we decide to drop support for ruby 1.9.3.

Outside of this pull request, I think we should look into whether we can reduce the `IdentityCache.should_cache?` checks to a single layer of abstraction rather than having it duplicated across multiple layers (e.g. IdentityCache and QueryApi).