incomplete regression fix for FormHelper selection - this resolves it for good

As suspected due to my local tests with in_array() our fix for master and 2.2 is incomplete!

    $options = array(1 => 'One', 2 => 'Two', 3 => 'Three', '3x' => 'Stringy');
    $disabled = array('2', '3x');

would still disable three fields instead of two (also `3 => 'Three'`).
I am working on a complete solution right now.

As I suspected, "(string)" cast in the in_array() is necessary here to avoid backfiring.
All tests pass now.

Also: When refactoring the form helper and "required/disabled" we might only have taken into account "multiple checkboxes".
But "multiple select" does not work. This fixes it.
Also added tests.