Add a helpful warning

Because almost every property on a class definition is a function, it's
easy to accidentally say this out of habit:

````javascript

App.MyController = Ember.Controller.extend({
  actions: function() {
    doThing: function() { ... }
  }
});

````

When you meant to say this:

````javascript

App.MyController = Ember.Controller.extend({
  actions: {
    doThing: function() { ... }
  }
});

````

This doesn't product any errors or warning, it just makes your handlers
invisible. My change adds a warning.

Fix overly sensitive unit test

As written, this unit test will fire erroneously whenever someone
introduces an unrelated Ember.warn statement in a codepath that gets
triggered during the test (even if that warning never triggers).

unit test for new ActionHandler warning