Don't freeze any other values than plain objects and arrays (#577)

source/deep-freeze.js

@@ -3,7 +3,7 @@ const is = require('@sindresorhus/is');
 
 module.exports = function deepFreeze(object) {
 	for (const [key, value] of Object.entries(object)) {
-		if (is.object(value)) {
+		if (is.plainObject(value) || is.array(value)) {
 			deepFreeze(object[key]);
 		}
 	}

test/create.js

@@ -1,3 +1,4 @@
+import http from 'http';
 import {URL} from 'url';
 import test from 'ava';
 import got from '../source';
@@ -145,6 +146,14 @@ test('no tampering with defaults', t => {
 	t.is(instance2.defaults.options.baseUrl, 'example');
 });
 
+test('only plain objects are freezed', async t => {
+	const instance = got.extend({
+		agent: new http.Agent({keepAlive: true})
+	});
+
+	await t.notThrowsAsync(() => instance(s.url));
+});
+
 test('defaults are cloned on instance creation', t => {
 	const options = {foo: 'bar', hooks: {beforeRequest: [() => {}]}};
 	const instance = got.create({options});