Fix Array.prototype.concat

The following tests failed in the test262 test suite due to incomplete
implementation:
- ch15/15.4/15.4.4/15.4.4.4/15.4.4.4-5-c-i-1
- ch15/15.4/15.4.4/15.4.4.4/S15.4.4.4_A2_T1
- ch15/15.4/15.4.4/15.4.4.4/S15.4.4.4_A2_T2
- ch15/15.4/15.4.4/15.4.4.4/S15.4.4.4_A3_T1

Change-Id: I423e77fe3d34140a08c61efdc18c81ef251bc927
Reviewed-by: Lars Knoll <lars.knoll@digia.com>