Update migx.class.php

Makes it possible to use (array saved) listbox-multiple fields (defined in migx config, not with a native tv) together with find/find_in_set. The problem was, that it tried to split an already existing array by comma, which resulted in nothing instead of the array that was passed to the function for checking
Merge pull request #149 from exside/patch-5

Update migx.class.php