GetSigniature fails on nested generic types

### Description
This is the same as #29 but for the GetSigniature extension method.

```csharp
class A
{
    class B<D>
    {
        class C<E> {  }
    }
}

typeof(A.B<int>.C<string>).GetSigniature();
typeof(A.B<>.C<>).GetSigniature();
```
It either throws an `InvalidOperationException` or returns too many generics for the nested generic signiature.