Alter Marionette.normalizeUIKeys to return a new/non-mutated hash

Instead of mutating the original argument hash, create and return a new cloned hash
Addresses issue #1728
In regard to your comment: "looks like travis found something: https://travis-ci.org/marionettejs/backbone.marionette/builds/32811496", read my comment above about the above failed test case. I'm not sure if I should refactor the existing test, or alter the code in `_delegateDOMEvents`.