Error with md-modal and Ember 2.12

[Gorzas]

I've found the following error using md-modal and Ember 2.12 in md-modal:

Uncaught TypeError: Cannot add property 2, object is not extensible

It seems that it fails when you add a fixed footer in a md-modal because the computed property tries to change the property that is observing (the array modalClassNames).

I'm going to send a PR with the changes ASAP.