Add a special case for constructing Headers with Headers (node-fetch#253)

Fixes: node-fetch#251.

Author: gajus

src/headers.js

@@ -34,6 +34,19 @@ export default class Headers {
 	constructor(init = undefined) {
 		this[MAP] = Object.create(null);
 
+		if (init instanceof Headers) {
+			const rawHeaders = init.raw();
+			const headerNames = Object.keys(rawHeaders);
+
+			for (const headerName of headerNames) {
+				for (const value of rawHeaders[headerName]) {
+					this.append(headerName, value);
+				}
+			}
+
+			return;
+		}
+
 		// We don't worry about converting prop to ByteString here as append()
 		// will handle it.
 		if (init == null) {

src/response.js

@@ -23,6 +23,7 @@ export default class Response {
 		this.url = opts.url;
 		this.status = opts.status || 200;
 		this.statusText = opts.statusText || STATUS_CODES[this.status];
+
 		this.headers = new Headers(opts.headers);
 
 		Object.defineProperty(this, Symbol.toStringTag, {

test/test.js

@@ -1205,6 +1205,18 @@ describe('node-fetch', () => {
 		});
 	});
 
+	it('should return all headers using raw()', function() {
+		url = `${base}cookie`;
+		return fetch(url).then(res => {
+			const expected = [
+				'a=1',
+				'b=1'
+			];
+
+			expect(res.headers.raw()['set-cookie']).to.deep.equal(expected);
+		});
+	});
+
 	it('should allow iterating through all headers with forEach', function() {
 		const headers = new Headers([
 			['b', '2'],