Make form field display valid values

In the current tip I see that there's a form field that can be used to enter array values.  But when array values are displayed by that form field, they come out as strings that look like "[u'a', u'b', u'c']".  These don't get parsed properly by the delimiter-based parsing in the field, so you'll get corruption if you view a record in the admin and resave it.  This patch fixes that by adding a prepare_value() method to ArrayFormField, which will dump the array contents out to a string using the delimiter that was given in __init__().