Fix no attribute 'validate_response' (#251)

Author: luolingchun

flask_openapi3/view.py

@@ -204,10 +204,7 @@ def register(
         for rule, (cls, methods) in self.views.items():
             for method in methods:
                 func = getattr(cls, method.lower())
-                if func.validate_response is not None:
-                    _validate_response = func.validate_response
-                else:
-                    _validate_response = self.validate_response
+                _validate_response = getattr(func, "validate_response", None) or self.validate_response
                 header, cookie, path, query, form, body, raw = parse_parameters(func, doc_ui=False)
                 view_func = app.create_view_func(
                     func,