Custom serializers are not passed the context obj in the deserializr

[alexggordon]

Currently, when using a custom serializer, the custom function does not result in the context being passed to whatever functions are provided to it.

We should change this, so we can allow for deserializer's to receive the context object

custom(
  // serializer, this works
  (jsonVal, done, context) => context.json.otherValue ? jsonVal : someOtherValue,

  // deserializer, context doesn't exist
  (jsonVal, done, context) => { console.log(context) // not here; return jsonVal }
) data;