[C++][Python] NumPy buffer sets is_mutable_ to true but does not set mutable_data_ when the NumPy array is writable

NumPyBuffer::NumPyBuffer(PyObject* ao) : Buffer(nullptr, 0) {
  PyAcquireGIL lock;
  arr_ = ao;
  Py_INCREF(ao);

  if (PyArray_Check(ao)) {
    PyArrayObject* ndarray = reinterpret_cast<PyArrayObject*>(ao);
    data_ = reinterpret_cast<const uint8_t*>(PyArray_DATA(ndarray));
    size_ = PyArray_SIZE(ndarray) * PyArray_DESCR(ndarray)->elsize;
    capacity_ = size_;

    if (PyArray_FLAGS(ndarray) & NPY_ARRAY_WRITEABLE) {
      is_mutable_ = true;
    }
  }
}