Throw exception on invalid arguments for pushObjects method (issue #2848)

Ember.MutableArray and Ember.ArrayProxy method #pushObjects should accept only
Ember.Enumerable or Array as argument. In other case it throws an exception.

This is a bugfix for: https://github.com/emberjs/ember.js/issues/2848

Pull request consist of: bugfix, changes in documentation, tests.
Merge pull request #2950 from chrmod/pushObjects

Throw exception on invalid arguments for pushObjects method (issue #2848)