title: Can't write non current user objects by PFUser currentuser
body: <p>I would like write a <code>PFUser</code> object by the <code>currentUser</code>, i've added the <code>ACL</code> based on the Parse developer guide, but i still get an error:</p>
  
  <pre><code> 'User cannot be saved unless they have been authenticated via logIn or signUp'    
  </code></pre>
  
  <p>_ My code:</p>
  
  <pre><code>PFQuery *query = [PFUser query];
      [query whereKey:@"username" equalTo:self.bBo];
      PFObject *friendData = [query getFirstObject];
  
      PFUser *user = (PFUser *)friendData;
  
      PFACL *userACL = [PFACL ACL];
  
      user.ACL = userACL;
  
      [userACL setWriteAccess:YES forUser:[PFUser currentUser]];
  
      PFRelation *friendRelation = [user relationforKey:@"array"];
      [friendRelation addObject:[PFUser currentUser]];
  
      [user saveInBackgroundWithBlock:^(BOOL succeeded, NSError *error) {
          if (error){
              NSLog(@"Error %@ %@", error, [error userInfo]);
          }
      }];
  </code></pre>
  
  <p>I think i did everything correct, so i can't figure out what can be the problem. So if you made earlier something like this or know where is the problem, i would really appreciate any suggestions. </p>