Membership Custom Field Values not Saved on Update

When an existing membership record is saved - if there are custom fields setup which extend the membership - the custom fields values are NOT saved.

(Custom fields for a membership ARE saved properly when a new membership is ADDED).

To recreate:

• Create a custom data group which Extends "Membership" (type = any)
• Add a text and a boolean (yes/no) field to the group
• Go to a contact with an existing membership and edit the custom field values and Save
• Go back to Edit screen - values are not saved.
  (If you create a New membership - custom values ARE saved.)

If the fix for this is simple - we should backport it to 1.7.