Allow filtering on (Auth-)headers in the new 'cache rules'-feature

Loving the continuous updates, especially the new Cache Rules-feature. I heavily (ab)used the Page-rule feature before, and this is exactly what I needed! However, it would be awesome if I could filter on headers in my expressions. For example I’d like to set an expression checking if there’s a header named Authorization present. I currently achieve similar results in a much dirtier way, and think implementing this will make the feature more elegant.