Replace reject_blocked with reject_blocked! in skipped callbacks.

What does this MR do?

After testing with Rails 5 I noticed there are some methods that are skipped and don't actually exist.

In Rails 4.2 and below, skipping methods in callbacks (skip_before_action, skip_after_action, etc.) where the method does not exist is "fine". Rails 5, however, throws a fit. See https://github.com/rails/rails/pull/19029

reject_blocked! was introduced in c9def945, I can't find any references to reject_blocked ever existing.

Are there points in the code the reviewer needs to double check?

Is there any reason to make a reject_blocked method? Anybody have any idea why this happened?

Why was this MR needed?

This throws errors in Rails 5, and is probably an oversight.

cc: @rspeicher @DouweM @dzaporozhets