Make sure it merges instances in order, omits Got defaults

Fixes #1450

test/normalize-arguments.ts

@@ -133,3 +133,20 @@ test('maxRetryAfter is calculated seperately from request timeout', t => {
 
 	t.is(options.retry.maxRetryAfter, 300);
 });
+
+test('extending responseType', async t => {
+	const instance1 = got.extend({
+		prefixUrl: 'https://localhost',
+		responseType: 'json'
+	});
+
+	const instance2 = got.extend({
+		headers: {
+			'x-test': 'test'
+		}
+	});
+
+	const merged = instance1.extend(instance2);
+
+	t.is(merged.defaults.options.responseType, 'json');
+});