contribution_page_id missing in contribution api result

The api does not return the contribution_page_id for a contribution.

Appears to affect all releases of version 3 api.

This can be seen via the api explorer
http://drupal.demo.civicrm.org/civicrm/api/explorer#/civicrm/ajax/rest?entity=Contribution&action=get&debug=1&sequential=1&json=1

Not sure why it would be omitted.

It can be worked around by making a call with contribution_page_id as the explicit return value.

Documenting here in cased any one else trips over this.
Issue was first reported here:
http://forum.civicrm.org/index.php?topic=25672.0