Eo mixin is converted to C# interface and C# concrete class.
When the mixin's method is called, the delegate function of Eo mixin's
C# concrete class is called.
Now, the delegate function of Eo mixin's C# concrete class calls C#
method with casting to Eo mixin's C# concrete class type.
If a user defined C# class implements Eo mixin's C# interface, the
implemented method cannot be called because the user defined C# class
type is not the same as Eo mixin's C# concrete class.
To resolve the above issue, the type casting code is fixed.