CRM-4579 Incorrect custom data value is set if default value is not specified.

steps to replicate.
1. create a custom data group
2. create/update some contacts w/  custom data.
3. create new field in given custom data group with no default option. ( like checkbox .. )
4. check value for civicrm_custom_field.default_value as well as custom value table for newly created custom field.
It should be null but it is showing null with value separator.

do check here : http://forum.civicrm.org/index.php/topic,8280.0/topicseen.html