Allowing query strings to be passed into dialogs

Bit of an edge case here, but I was trying to set up a mechanism where a references dialog form would have field values prepopulated from data contained in the parent form (potentially unsubmitted data hence would need to be done via javascript).

I wrote a behaviour that appended a query string to the 'add [node type]' href attribute, but unfortunately, the code in Drupal.ReferencesDialog.open appends it's own query string without checking for a pre-existing one.

I've put together a patch (attached), but this is the first patch I've ever submitted so apologies if I've done it horribly wrong.

Thanks,
Paul.