Add two unit tests: (#769)

1. test loggin in with the js sdk to verify documentation change
2. add a negative test that proves we need the master key to link

Author: acinader

integration/test/ParseUserTest.js

@@ -619,6 +619,30 @@ describe('Parse User', () => {
     expect(user._isLinked(provider)).toBe(false);
   });
 
+  it('linked account can login with authData', async () => {
+    const user = new Parse.User();
+    user.setUsername('Alice');
+    user.setPassword('sekrit');
+    await user.save(null, { useMasterKey: true });
+    await user._linkWith(provider.getAuthType(), provider.getAuthData(), { useMasterKey: true });
+    expect(user._isLinked(provider)).toBe(true);
+    expect(user.authenticated()).toBeFalsy();
+    Parse.User.enableUnsafeCurrentUser();
+    const loggedIn = await Parse.User.logInWith(provider.getAuthType(), provider.getAuthData());
+    expect(loggedIn.authenticated()).toBeTruthy();
+  });
+
+  it('linking un-authenticated user without master key will throw', async (done) => {
+    const user = new Parse.User();
+    user.setUsername('Alice');
+    user.setPassword('sekrit');
+    await user.save(null, { useMasterKey: true });
+    user._linkWith(provider.getAuthType(), provider.getAuthData())
+      .then(() => done.fail('should fail'))
+      .catch(e => expect(e.message).toBe(`Cannot modify user ${user.id}.`))
+      .then(done);
+  });
+
   it('can link with custom auth', async () => {
     Parse.User.enableUnsafeCurrentUser();
     const provider = {