JS: Add test with header access

javascript/ql/test/library-tests/frameworks/ClientRequests/ClientRequests.expected

@@ -192,3 +192,4 @@ test_getAResponseDataNode
 | tst.js:151:5:151:23 | superagent.get(url) | tst.js:151:30:151:32 | err | error | false |
 | tst.js:151:5:151:23 | superagent.get(url) | tst.js:151:35:151:37 | res | stream | false |
 | tst.js:160:5:160:17 | xhr.send(url) | tst.js:162:9:162:29 | xhr.get ... eJson() | json | false |
+| tst.js:160:5:160:17 | xhr.send(url) | tst.js:163:9:163:32 | xhr.get ... aders() | headers | false |

javascript/ql/test/library-tests/frameworks/ClientRequests/tst.js

@@ -160,5 +160,6 @@ import {ClientRequest, net} from 'electron';
     xhr.send(url);
     xhr.addEventListener('readystatechange', function() {
         xhr.getResponseJson();
+        xhr.getResponseHeaders();
     });
 })