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

.codeclimate.yml

@@ -1,3 +1,5 @@
 exclude_patterns:
 - docs
 - test/specs
+- test/client-specs
+- test/server-specs

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);
 		});