EventSource: test U+0000 in ID better

See https://bugzilla.mozilla.org/show_bug.cgi?id=1387355 for context.

eventsource/format-field-id-null.window.js

@@ -0,0 +1,25 @@
+[
+  "\u0000\u0000",
+  "x\u0000",
+  "\u0000x",
+  "x\u0000x",
+  " \u0000"
+].forEach(idValue => {
+  const encodedIdValue = encodeURIComponent(idValue);
+  async_test(t => {
+    const source = new EventSource("resources/last-event-id.py?idvalue=" + encodedIdValue);
+    t.add_cleanup(() => source.close());
+    let seenhello = false;
+    source.onmessage = t.step_func(e => {
+      if (e.data == "hello" && !seenhello) {
+        seenhello = true;
+        assert_equals(e.lastEventId, "");
+      } else if(seenhello) {
+        assert_equals(e.data, "hello");
+        assert_equals(e.lastEventId, "");
+        t.done();
+      } else
+        assert_unreached();
+    });
+  }, "EventSource: id field set to " + encodedIdValue);
+});