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

Not sure where the comment went but...

Here's the reason why I added the above changes to marionette.view. Let me know if I'm missing something.

In regards to normalizeUIKeys, _delegateDOMEvents is taking two paths:
1. If passing in an eventsArg argument, only normalize the eventsArg, not the this.events property.
2. If passing in an undefined eventsArg argument, normalize the this.events property.

Focusing on #2 (eventsArg = undefined), originally normalizeUIKeys would mutate the passed-in hash. The new behavior of normalizeUIKeys returns a non-mutated/new hash. Since the events variable is assigned this new parsed hash, this.events will not be updated with the new assignment. Hence the if(_.isUndefined(eventsArg)) {this.events = events;} which is stating that since eventsArg is undefined, deal with this.events (path #2).