FIX: fetch-mock Response.body is not always a stream

The WhatWG fetch spec requires that Body.body (and therefore Response.body)
is always a readable stream, but it doesn't always happen that way in Node.
This seems to be because ResponseBuilder tries to fetch the stream module
from fetchMock, but it looks in the wrong place.

Fixes wheresrhys#609

src/lib/response-builder.js

@@ -137,9 +137,12 @@ e.g. {"body": {"status: "registered"}}`);
 
 		// On the server we need to manually construct the readable stream for the
 		// Response object (on the client this done automatically)
-		if (this.Stream) {
+		if (
+			this.fetchMock.Stream &&
+			!(this.body instanceof this.fetchMock.Stream.Readable)
+		) {
 			this.debug('Creating response stream');
-			const stream = new this.Stream.Readable();
+			const stream = new this.fetchMock.Stream.Readable();
 			if (this.body != null) { //eslint-disable-line
 				stream.push(this.body, 'utf-8');
 			}

test/server-specs/server-only.test.js

@@ -40,6 +40,28 @@ describe('nodejs only tests', () => {
 			});
 		});
 
+		it('always responds with a readable stream', (done) => {
+			const { Writable } = require('stream');
+			const write = sinon.stub().callsFake((chunk, enc, cb) => {
+				cb();
+			});
+			const writable = new Writable({
+				write,
+			});
+
+			fetchMock.mock(/a/, Buffer.from('response string', 'utf8'), {
+				sendAsJson: false,
+			});
+			fetchMock.fetchHandler('http://a.com').then((res) => {
+				res.body.pipe(writable);
+			});
+
+			writable.on('finish', () => {
+				expect(write.args[0][0].toString('utf8')).to.equal('response string');
+				done();
+			});
+		});
+
 		// See https://github.com/wheresrhys/fetch-mock/issues/575
 		it('can respond with large bodies from the interweb', async () => {
 			const fm = fetchMock.sandbox();

test/specs/config/constructors.test.js

@@ -130,7 +130,7 @@ describe('custom implementations', () => {
 			expect(res.isFake).to.be.true;
 			expect(spiedReplacementResponse.callCount).to.equal(1);
 			const lastCall = spiedReplacementResponse.lastCall.args;
-			expect(lastCall[0]).to.equal('hello');
+			expect(lastCall[0]).to.have.property(Symbol.asyncIterator);
 			expect(lastCall[1].status).to.equal(200);
 			expect(defaultSpies.Response.callCount).to.equal(0);
 		});