Switch to OmniString::explode implementation

Summary:
230faccc628f has introduced a correct reference implementation
of explode, without any "returns false" surprise if the delimiter
is "".

This change switches OmniArray::explode implementation to use that.

Test Plan: Unit tests provided

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2283