Fix {{yield}} helper to work with non Ember.Component views that have layouts

This adds a fix and test for #3075, which is actually a much worse bug than I thought - currently on master, layouts are entirely broken as they change the context of the child view to be the  context of the parent view. 

I also made another jsfiddle that's simpler: http://jsfiddle.net/6hWjz/5/