Combine parent child serializer.

[chyipin]

Extend child serializer to merge serialized data for parent serializer with same fields.

Currently, child serializer for specific fields will override the parent serializer. This prevents custom serialization at parent level.
Parent:

        serializers: {
            req: function (req) {
              return {
                username: req.user.username
              };
            }
        }

Child:

        serializers: {
            req: function (req) {
              return {
                // other fields: timers, stats, etc
              };
            }
        }

Primary use case is restify audit logger.
The proposed changes applies the serializers from parent and child, and merge the serialized result preferring the data from the child if there are field collision (similar to a simple `_.merge). If there happens to be a clash in field names, the fix would be to change the field name for the serializer in the parent.