:equal rule support for MultiSelect

If multiple values are selected `equal` rule always returns false.

$allowed = [1,2,3];
$values = [1]; // => TRUE
$values = [1,5]; // => FALSE

Provided code little changes behaviour from `equal` to `contains|intersects`
$allowed = [1,2,3];
$values = [1]; // => TRUE
$values = [1,5]; // => TRUE

Not sure if this is acceptable or another rule keyword should be used instead.