Consistent casing of response keys

Every response now goes through CamelCaseHelper::keysToCamelCase() function because relying on only the model mapper caused inconsistencies (if the model contained a property that is defined just as an object, not a specific model then the keys of this object were not camel-cased).

Also, if json_encode() is run over the response then everything is now consistently snake_cased (as in the raw response)