message: Add double quotes to filter PaymentType
  Without double quotes around ACCRECPAYMENT getPayments returns `<Message>No property or field \'ACCRECPAYMENT\' exists in type \'Payment\'</Message>`
diff: @@ -97,7 +97,7 @@ This example shows how to retrieve an payment using the 'where' filter.
  ```javascript
  //filter payments that are type ACCREC
  -var filter = 'PaymentType == ACCRECPAYMENT';
  +var filter = 'PaymentType == "ACCRECPAYMENT'';
  xeroClient.core.payments.getPayments({where: filter})
  .then(function(payments) {
  