Fix CRM_Core_BAO_CustomGroup::validateSubTypeByEntity()

Fix regression introduced in commit: https://github.com/civicrm/civicrm-core/commit/79363422d47677e58596798ed26304c8c4b9953c.

`civicrm_api3('Contact', 'getoptions')` return an associative array. Here, we want to check the key values, so we need to add an `array_keys()` before `in_array`.

Olivier;