Fix Promise#finaly shim regression and cover with tests

tests/polyfills.test.js

@@ -0,0 +1,15 @@
+import expect from 'unexpected';
+
+describe('promise.prototype.finally', () => {
+    it('should be invoked after promise resolved', () =>
+        expect(new Promise((resolve) => {
+            Promise.resolve().finally(resolve)
+        }), 'to be fulfilled')
+    );
+
+    it('should be invoked after promise rejected', () =>
+        expect(new Promise((resolve) => {
+            Promise.reject().finally(resolve)
+        }), 'to be fulfilled')
+    );
+});