CRM-8169 Membership api should accept 'id' only - BAO issue

Part of the api standard is that 'create' shoud work if only 'id' is set - in the case of membership api it falls over on membership_log requiring a contact_id & doesn't have an adequate default. Attached patch will load up the existing contact_id a little bit earlier in the piece that happened previously & uses the contact_id in this case. I have a test which passes if this is in place. This seems more efficient that loading the existing membership in the api & then again in the BAO

Just want someone to QA this