Generic implementations with only open types are not returned

Given interface

```
public interface IBar<T1, T2>
{
    void Method(T1 t1, T2 t2);
}
```

and implementation:

```
public class Foo<T1, T2> : IBar<T1, T2>
{
    public void Method(T1 t1, T2 t2)
    {
        throw new NotImplementedException();
    }
}
```

We should be returning this implementation. It isn't because we exclude it if all types are still open.
