make sure the default `highWaterMark` equals 16384

src/request.js

@@ -134,7 +134,7 @@ export default class Request extends Body {
 				input.compress : true);
 		this.counter = init.counter || input.counter || 0;
 		this.agent = init.agent || input.agent;
-		this.highWaterMark = init.highWaterMark || input.highWaterMark;
+		this.highWaterMark = init.highWaterMark || input.highWaterMark || 16384;
 	}
 
 	get method() {

test/main.js

@@ -1791,6 +1791,13 @@ describe('node-fetch', () => {
 		);
 	});
 
+	it('the default highWaterMark should equal 16384', () => {
+		const url = `${base}hello`;
+		return fetch(url).then(res => {
+			expect(res.highWaterMark).to.equal(16384);
+		});
+	});
+
 	it('should timeout on cloning response without consuming one of the streams when the second packet size is equal default highWaterMark', function () {
 		this.timeout(300);
 		const url = local.mockResponse(res => {