Convert attribute, node, child string keys/names to symbols

This is a PR in reference to this issue https://github.com/nesquena/rabl/issues/549. Fixes inconsistencies with how different JSON engines deal with duplicate keys as strings and symbols. Most of the faster JSON engines I tested (Yajl and OJ) don't care and will let these pass through. ActiveSupport::JSON is the outlier that will de-duplicate by converting all keys to symbols first.