Bug-fix: set this.eventToCallbackMap property values to undefined instead of null on destroy()

[rovolution]

Fixes #640

[seiyria]

Looks good to me. Maybe a semantics thing, but would it make more sense to delete it instead of waiting for it to get gc'd?

[rovolution]

delete just removes the reference and marks it for GC on the subsequent GC pass. Won't make much of a difference other than removing the eventName key from the this. eventToCallbackMap object

[rovolution]

@seiyria went ahead and refactored to use delete. feels great to have a good unit test that validates the implementation change still achieves the desired functionality :)

[rovolution]

@snurby7 merged and published to 9.4.0! thanks again!