Parse::Object shouldn't extend Hash

[ericcj]

it should duck-type/delegate [] and []= to a hash of estimated_data or something but it shouldn't actually return true for is_a?(Hash) since there's a bunch of Hash methods that don't make sense on it. there's too much dangerous code out there to extend hash and hope things work, e.g. https://github.com/rails/rails/blob/v4.1.1/activesupport/lib/active_support/json/encoding.rb#L85 causes json encoding a Parse::Object to not use its rest_api_hash that we intend and thereby be missing its className

[xavdid]

This is a fairly valid issue, but it's also a big code change. We'll keep it on the radar.