Union displays do not use the exposed filters

Your solution didn't quite work, because we also need to call the setExposedInput() method to copy the user-submitted form data into the union display.

I also added code to do a sanity check to be sure that the union display doesn't have exposed form elements that are not present on the main view, as these won't appear to the user without me writing a bunch more code.

Automatically closed - issue fixed for 2 weeks with no activity.