Fix clearTimeout in web workers

Closes #1732. This worked fine in Node since there clearTimeout does not care what its thisArg is. But in browsers, it does, causing the error described there.

lib/jsdom/browser/Window.js

@@ -534,15 +534,17 @@ function startTimer(window, startFn, stopFn, timerId, callback, ms, args) {
 function stopTimer(window, id) {
   const timer = window.__timers[id];
   if (timer) {
-    timer[1].call(window, timer[0]);
+    // Need to .call() with undefined to ensure the thisArg is not timer itself
+    timer[1].call(undefined, timer[0]);
     delete window.__timers[id];
   }
 }
 
 function stopAllTimers(window) {
   Object.keys(window.__timers).forEach(key => {
     const timer = window.__timers[key];
-    timer[1].call(window, timer[0]);
+    // Need to .call() with undefined to ensure the thisArg is not timer itself
+    timer[1].call(undefined, timer[0]);
   });
   window.__timers = Object.create(null);
 }

test/jsdom/inside-worker-smoke-tests.js

@@ -84,4 +84,17 @@ describe("jsdom/inside-worker-smoke-tests", () => {
       }
     });
   });
+
+  specify("clearTimeout (GH-1732)", { async: true }, t => {
+    const document = jsdom.jsdom(`<script>const t = setTimeout(() => {
+      clearTimeout(t);
+      window.done();
+    }, 100);</script>`);
+
+    const window = document.defaultView;
+    window.addEventListener("error", e => {
+      assert.ifError(e.error);
+    });
+    window.done = () => t.done();
+  });
 });