CRM-15542 - Unit test.

A unit test for CRM-15542. Technically this was a bug in the BAO layer, not in the API.
But I felt more comfortable writing this test for the API. You can merge this if it is useable.
----------------------------------------
* CRM-15542: The api ignores option_group_id when creating a new custom_field.
  https://issues.civicrm.org/jira/browse/CRM-15542