Prevent 'amount' from being misinterpreted (#924)

The 'amount' field should almost never be used.
With partial payments, the field can show an amount that is
significantly less than the amount that the transaction actually
delivered. This change sets amount to 0 XRP when it may be misleading.

This change omits the `amount` when parsing payment transactions.
See `HISTORY.md` for recommended alternatives.

test/fixtures/responses/index.js

@@ -35,6 +35,7 @@ module.exports = {
     orderSell: require('./get-transaction-order-sell.json'),
     noMeta: require('./get-transaction-no-meta.json'),
     payment: require('./get-transaction-payment.json'),
+    paymentIncludeRawTransaction: require('./get-transaction-payment-include-raw-transaction.json'),
     settings: require('./get-transaction-settings.json'),
     trustline: require('./get-transaction-trustline-set.json'),
     trackingOn: require('./get-transaction-settings-tracking-on.json'),
@@ -68,6 +69,7 @@ module.exports = {
   },
   getTransactions: {
     normal: require('./get-transactions.json'),
+    includeRawTransactions: require('./get-transactions-include-raw-transactions.json'),
     one: require('./get-transactions-one.json')
   },
   getTrustlines: {