Error using union types in interface

Minimal reproduction:
```js
// @flow
interface Foo {
type: string | string[];
}
function bar(): Foo {
return { type: 'hello' };
}
```
Expected behavior: Flow type-checks without issue.

Actual behavior: Error
```
test.js:3
3: type: string | string[];
^^^^^^^^ array type. This type is incompatible with the expected return type of
6: return { type: 'hello' };
^^^^^^^ string
```
Flow version: 0.47.0