Intermediate pending transaction does not get renewed / updated properly

Its a side-affect of improvement done in CRM-8141.

Case :
Member had an expired membership.
Tried to renew, but transaction did not complete leaving a pending transaction of a new membership type
Tried to renew again and did complete the transaction.

When pending membership type is completed via 'update pending contribution' action, the membership is extended based on latest membership type and not based on type that was pending.