ArrayObject::append() should delegate to ArrayObject::offsetSet()

Summary: Match php5 implementation by delegating rather than direct manipulation of internal state.
Fixes issue #3403.
Closes https://github.com/facebook/hhvm/pull/3404

Reviewed By: @fredemmott

Differential Revision: D1486300

Pulled By: svcscm
Change Facebook proxy information for HHVM OSS test runner

Summary: Facebook: https://fb.facebook.com/groups/pe/permalink/815638725137542/

Reviewed By: @JoelMarcey

Differential Revision: D1500568