Fix dequeue not calling for UICollectionReusableView

[AgapovOne]

Declaring CollectionHeaderView<x>.on.dequeue doesn't get called inside any code of a framework. Easily can be tested with adding on.dequeue in ExampleApp CollectionExampleController.swift.

This pull requests fixes this issue. Probably this is not the proper way to get dispatcher. Correct me if I'm wrong. But that needs to be fixed asap