Add concat() test using ArrayProxy

[Turbo87]

The Ember.Array interface does not define a concat() method, but the concat() macro seems to use it as if it exists. We might have an issue here and we should at least add a test to make sure whether it works or not.

[kellyselden]

Agreed. Anyone watching is free to help. @Turbo87 Can now help review and merge.

[baljeet03]

@Turbo87 I would like to do that. I had a little doubt regarding the test case though. Should I create a Ember.ArrayProxy object and convert it to array and test it or something else.