Incorrect binding when the derived class is polymorphic and the base is not

The issue from Gitter

Reproducible example code

#include "pybind11/pybind11.h"
namespace py = pybind11;

struct BaseNonPolymorphic {
	unsigned int a;
	unsigned int b;
	unsigned int c;
	unsigned int d;
};
struct Derived : BaseNonPolymorphic {
	Derived() {
		a = 1;
		b = 2;
		c = 3;
		d = 4;
	};
	virtual ~Derived() {};
};
PYBIND11_MODULE(test_inheritance, m) {
	py::class_<BaseNonPolymorphic > base_class(m, "Base");
	base_class
		.def_readwrite("d", &BaseNonPolymorphic::d)
	;

	py::class_<Derived> derived_class(m, "Derived", base_class);
	derived_class
		.def(py::init<>())
	;
}

import test_inheritance
derived_instance = test_inheritance.Derived()
assert derived_instance.d == 4

Start with the C++ reproducible example using pybind11/pybind11.h and the PYBIND11_MODULE entry point, then run the Python assertion against the Derived binding. Trace how the non-polymorphic base and polymorphic derived class are bound; done means derived_instance.d evaluates to 4 as shown.