[BUGFIX beta] camelize before singularize to apply uncountability properly

This is an issue with the typeForRoot function of the RestSerializer not properly accounting for uncountable keys (snake cased keys as expected by the serializer, or underscore prefixed as used by the embedded records mixin). By normalizing the key first using camelize, we can then properly transform it using singularize which can then recognize any values that have been marked as "uncountable".

@igorT