Default Response status is 200 OK

This is in accordance with Section 6.4 of the spec:

    dictionary ResponseInit {
      unsigned short status = 200;
      ByteString statusText = "OK";
      HeadersInit headers;
    };

Fixes #376

fetch.js

@@ -357,9 +357,9 @@
     }
 
     this.type = 'default'
-    this.status = options.status
+    this.status = 'status' in options ? options.status : 200
     this.ok = this.status >= 200 && this.status < 300
-    this.statusText = options.statusText
+    this.statusText = 'statusText' in options ? options.statusText : 'OK'
     this.headers = new Headers(options.headers)
     this.url = options.url || ''
     this._initBody(bodyInit)

test/test.js

@@ -464,6 +464,13 @@ suite('Request', function() {
 
 // https://fetch.spec.whatwg.org/#response-class
 suite('Response', function() {
+  test('default status is 200 OK', function() {
+    var res = new Response()
+    assert.equal(res.status, 200)
+    assert.equal(res.statusText, 'OK')
+    assert.isTrue(res.ok)
+  })
+
   // https://fetch.spec.whatwg.org/#concept-bodyinit-extract
   suite('BodyInit extract', function() {
     featureDependent(suite, support.blob, 'type Blob', function() {