Querying same field w/ and w/o attributes for different union types results in error

Query:

```
{
  curatedList {
     ...on Program {
        title(lang: "fi")
     }

     ...on Article {
        title
     }
  }
}
```

Error:

```
Fields title conflict because they have differing arguments.
```

Should it really work like that, or is this a bug?

Article and Program are both members of union type Content
