CRM-11662 Add missing logic to copy case custom fields in CRM_Case_BAO_C...

...ase::mergeCases.

Note that the issue with case activity custom fields not being copied forwarded during merge seems to have been fixed w/ previous refactoring. I could not recreate that problem.
----------------------------------------
* CRM-11662:
  http://issues.civicrm.org/jira/browse/CRM-11662