Add test that shows parsing issue

It's meant to show the pipelining issue. I guess we'll solve the
newly-discovered parsing issue first.

src/connection.hpp

@@ -128,7 +128,7 @@ struct Connection : MessageSender {
    * ```
    */
   void append_message(std::span<std::string_view> data_parts, Arena arena,
-                      bool close_after_send = false) override;
+                      bool close_after_send) override;
 
   /**
    * @brief Get a WeakRef to this connection for async operations.