Fix call to get_declared_fields: pass dict_cls

Pass dict_cls even if a defaut was added to get_declared_fields, for
compatibility with classes overriding get_declared_fields with no
default value.

src/marshmallow/schema.py

@@ -117,6 +117,7 @@ def __new__(mcs, name, bases, attrs):
             klass=klass,
             cls_fields=cls_fields,
             inherited_fields=inherited_fields,
+            dict_cls=dict,
         )
         return klass