CRM-12976 : used first option mentioned in CRM-12976#comment-51647

This patch solves : 
 - the membership type editing / updating with its amount with value 0.00 . (https://test.civicrm.org/job/CiviCRM-4.3-git/LABEL=debian6,SUITE=api_v3_AllTests/274/ test now passes)
 - the CRM-12976 issue is also remains fixed by it.
 - also CRM-11582 doesn't break.