Requires only the necessary core_ext from active_support.

base.rb seem to only require the hash extension but was requiring the
whole active_support/core_ext. That injected
active_support/core_ext/object/json.rb in the call chain.
When you have an object composed of other objects where the parent
doesn't define :to_json it would completely ignore :to_json defined
in the children, using active_support's instead.