Fix map filter on Hash inputs

Fixes breakage which showed up in Jekyll, see https://github.com/Shopify/liquid/pull/239#issuecomment-25845173 and https://github.com/mojombo/jekyll/issues/1620.

If we really want the new behaviour, we should wait for a new major release to introduce it.

Please review @arthurnn @dylanahsmith / cc @parkr