Type cast Version (and other 64-bit values in result.card object) to …

…prevent auto-retries of storeCard (#241)

**Please explain the changes you made here:**
We've noticed that the store-card-on-file example has been visibly
reporting failures. After some investigation, I realized that these
failures were not caused by malformed requests, but by server.js
auto-retrying (non-failing) requests. These delinquent retries
(correctly) failed with `source_used` errors. The retries were actually
caused by an exception in this call to [send within
storecCard](https://github.com/square/web-payments-quickstart/blob/042c73641a18f2b80a8d09d3cd09f5ad61871082/server.js#L121).
One of the values in result.card (specifically _result.card.version_) is
a 64-bit integer value, and "send" fails to encode this type. By casting this value (and other 64bit int values) to strings before we call "send", we can prevent the auto retries and no longer report failures.

_Does this close any currently open issues?_

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server.js

@@ -114,9 +114,11 @@ async function storeCard(req, res) {
 
       logger.info('Store Card succeeded!', { result, statusCode });
 
-      // remove 64-bit value from response
-      delete result.card.expMonth;
-      delete result.card.expYear;
+      // cast 64-bit values to string
+      // to prevent JSON serialization error during send method
+      result.card.expMonth = result.card.expMonth.toString();
+      result.card.expYear = result.card.expYear.toString();
+      result.card.version = result.card.version.toString();
 
       send(res, statusCode, {
         success: true,