serialize tuple enums with single element directly as the value instead of a sequence

serde_tests/tests/test_bytes.rs

@@ -39,6 +39,15 @@ impl serde::Serializer for BytesSerializer {
         Err(Error)
     }
 
+    fn visit_enum_simple<T>(&mut self,
+                            _name: &str,
+                            _variant: &str,
+                            _value: T,
+                            ) -> Result<(), Error>
+    {
+        Err(Error)
+    }
+
     fn visit_bool(&mut self, _v: bool) -> Result<(), Error> {
         Err(Error)
     }