Let Populator::get('foo[4]') work for array items

This simple change makes `Populator::get('foo[3]')` not throw an error if $this->items is an array (such as with `Former::populate(array('foo' => array('3' => true)))`).

I didn't add a test, since it would conflict with the tests in #223; I'll add additional tests after one or both of these issues are merged.