always set a signal on Request

fetch.js

@@ -370,7 +370,12 @@ export function Request(input, options) {
   }
   this.method = normalizeMethod(options.method || this.method || 'GET')
   this.mode = options.mode || this.mode || null
-  this.signal = options.signal || this.signal
+  this.signal = options.signal || this.signal || (function () {
+    if ('AbortController' in global) {
+      var ctrl = new AbortController();
+      return ctrl.signal;
+    }
+  }());
   this.referrer = null
 
   if ((this.method === 'GET' || this.method === 'HEAD') && body) {

test/test.js

@@ -1147,7 +1147,20 @@ exercise.forEach(function(exerciseMode) {
       })
 
       featureDependent(suite, exerciseMode !== 'native' || support.aborting, 'aborting', function() {
-        test('initially aborted signal', function() {
+        test('Request init creates an AbortSignal without option', function() {
+          var request = new Request('/request')
+          assert.ok(request.signal);
+          assert.equal(request.signal.aborted, false);
+        })
+
+        test('Request init passes AbortSignal from option', function () {
+          var controller = new AbortController()
+          var request = new Request('/request', {signal: controller.signal})
+          assert.ok(request.signal);
+          assert.deepEqual(controller.signal, request.signal);
+        })
+
+        test('initially aborted signal', function () {
           var controller = new AbortController()
           controller.abort()