Allow undefined Response status

This allows the Response constructor to accept a status code that's
explicitly set to undefined. Native fetch implementations in Chrome and
Firefox behave this way.

The following now works, but would previously throw an error:

    new Response('', { status: undefined })

fetch.js

@@ -380,7 +380,7 @@
     }
 
     this.type = 'default'
-    this.status = 'status' in options ? options.status : 200
+    this.status = options.status === undefined ? 200 : options.status
     this.ok = this.status >= 200 && this.status < 300
     this.statusText = 'statusText' in options ? options.statusText : 'OK'
     this.headers = new Headers(options.headers)