Check default holder

-Recognize "std::unique_ptr<T, D>" as a default holder even if "D" doesn't match between base and derived holders

Author: trelau

include/pybind11/pybind11.h

@@ -1057,7 +1057,7 @@ class class_ : public detail::generic_type {
         record.holder_size = sizeof(holder_type);
         record.init_instance = init_instance;
         record.dealloc = dealloc;
-        record.default_holder = std::is_same<holder_type, std::unique_ptr<type>>::value;
+        record.default_holder = detail::is_instantiation<std::unique_ptr, holder_type>::value;
 
         set_operator_new<type>(&record);