Fix statusText: undefined should give '' and null should give 'null'

fetch.js

@@ -450,7 +450,7 @@ export function Response(bodyInit, options) {
   this.type = 'default'
   this.status = options.status === undefined ? 200 : options.status
   this.ok = this.status >= 200 && this.status < 300
-  this.statusText = 'statusText' in options ? options.statusText : ''
+  this.statusText = options.statusText === undefined ? '' : '' + options.statusText
   this.headers = new Headers(options.headers)
   this.url = options.url || ''
   this._initBody(bodyInit)

test/test.js

@@ -694,6 +694,16 @@ exercise.forEach(function(exerciseMode) {
         assert.equal(r.headers.get('content-type'), 'text/plain')
       })
 
+      test('construct with undefined statusText', function() {
+        var r = new Response('', {statusText: undefined})
+        assert.equal(r.statusText, '')
+      })
+
+      test('construct with null statusText', function() {
+        var r = new Response('', {statusText: null})
+        assert.equal(r.statusText, 'null')
+      })
+
       test('init object as first argument', function() {
         var r = new Response({
           status: 201,