Fix HTTP 417 error when testing with Node v5.5.0

In Node v5.5.0, the HTTP server became more diligent about sending a HTTP 417
Expectation Failed response if the `Expect` header was not `100-continue`.
Since `RequestHelper.httpOptions` was creating requests with an empty `Expect`
header, and did not define a `checkExpectation` event handler, the server
returned 417 errors for these requests.

Also updated `RequestHelper.sendRequest` to use the status code text as the
error message if the error response body is empty.

https://nodejs.org/en/blog/release/v5.5.0/
nodejs/node#4501
https://nodejs.org/api/http.html#http_event_checkexpectation
https://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.18
https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.20
https://www.w3.org/Protocols/rfc2616/rfc2616-sec8.html#sec8.2.3

test/request-helper.js

@@ -24,7 +24,7 @@ RequestHelper.prototype.httpOptions = function(port, payload, secret) {
     method: 'POST',
     headers: {
       'content-type': 'application/json',
-      'Expect': '',
+      'Expect': '100-continue',
       'User-Agent': 'GitHub-Hookshot/9db916b',
       'X-GitHub-Delivery': '01234567-0123-0123-1234-0123456789ab',
       'X-GitHub-Event': 'push',
@@ -71,7 +71,8 @@ RequestHelper.prototype.sendRequest = function(options, payload) {
         if (res.statusCode >= 200 && res.statusCode <= 300) {
           resolve(data);
         } else {
-          reject(new Error(data));
+          reject(new Error(
+            data.length !== 0 ? data : http.STATUS_CODES[res.statusCode]));
         }
       });
     });