Asst.getPayment returns "0.0" when no payment
Asst.getPayment returns "0.0" when no payment

Assigmnet.getPayment should return 0.0 when no payment is set instead of `undefined`

VirtualLab/turkserver-meteor,HarvardEconCS/turkserver-meteor,VirtualLab/turkserver-meteor,VirtualLab/turkserver-meteor,HarvardEconCS/turkserver-meteor,HarvardEconCS/turkserver-meteor,TurkServer/turkserver-meteor,TurkServer/turkserver-meteor,TurkServer/turkserver-meteor