Fixed: multiple instances of same class share computed.meta()

My last PR introduced a regression caused by the fact that Ember.computed() returns an instance of a ComputedProperty that is shared by all instances of a class where change-gated computed property is used.

This PR adds 2 new tests that test for this scenario and introduce a solution that passes these tests.
For esthetic reasons, but you're right it will, so I'll take it out.