Multiple Relationship types storage broken in membership type form (introduced in 3.3.1)

Check: http://forum.civicrm.org/index.php/topic,17054.0.html

A few things:

1. Users reported it stores the word Array, i could not reproduce. We might want to use the upgrade script to set the column to null if so. I could reproduce the direction just being stored as '_'

2. Also check that if u unselect the relationship type, the null value is stored

a patch for this is attached, please test and improve