Allow for custom meta controllers in restws_meta_controls

I currently have another issue in that shows how you can use an alter statement to add your own custom meta controller (#2024603: Pull more then just item id when using references (code snippet)).

This works fine when loading a single item since meta controllers aren't validated in this operation. If you try to query using custom methods, you run into an issue where custom controllers are invalid. This patch is simple and would allow for custom controllers which the developer would have to account for but should have no impact on the base module.