[FIX] source_slippage should default to zero

- A SendMax value is calculated using source_slippage, but that is an
  optional parameter.

- Typical client error:

```json
{
   "success": false,
   "error_type": "transaction",
   "error": "plus() not a number: undefined"
}
```

api/lib/rest-to-tx-converter.js

@@ -78,7 +78,7 @@ RestToTxConverter.prototype.convert = function(payment, callback) {
             && payment.source_amount.counterparty
             === payment.destination_amount.counterparty)) {
         var max_value = bignum(payment.source_amount.value)
-                        .plus(payment.source_slippage).toString();
+                        .plus(payment.source_slippage || 0).toString();
         if (payment.source_amount.currency === 'XRP') {
           transaction.sendMax(utils.xrpToDrops(max_value));
         } else {

test/unit/fixtures/rest-converter.js

@@ -116,7 +116,6 @@ module.exports.exportsPaymentRestIssuers = function(options) {
     destinationAccount: addresses.COUNTERPARTY,
     sourceIssuer: addresses.VALID,
     destinationIssuer: addresses.COUNTERPARTY,
-    sourceSlippage: '0',
     sourceValue: '10'
   });