Improve Body spec compliance when body is null

src/body.js

@@ -79,6 +79,11 @@ export default class Body {
 
 		this[DISTURBED] = true;
 
+		// body is null
+		if (!this.body) {
+			return Body.Promise.resolve(new Buffer(0));
+		}
+
 		// body is string
 		if (typeof this.body === 'string') {
 			return Body.Promise.resolve(convertBody([new Buffer(this.body)], this.headers));

src/response.js

@@ -17,7 +17,7 @@ import Body, { clone } from './body';
  * @return  Void
  */
 export default class Response extends Body {
-	constructor(body, opts = {}) {
+	constructor(body = null, opts = {}) {
 		super(body, opts);
 
 		this.url = opts.url;

test/test.js

@@ -1427,6 +1427,14 @@ describe('node-fetch', () => {
 		});
 	});
 
+	it('should default to null as body', function() {
+		const res = new Response();
+		expect(res.body).to.equal(null);
+		return res.text().then(result => {
+			expect(result).to.equal('');
+		});
+	});
+
 	it('should default to 200 as status code', function() {
 		const res = new Response(null);
 		expect(res.status).to.equal(200);