so_5::details::make_message_instance_impl metafunction doesn't set message mutability flag properly

[eao197]

It seems that make_message_instance_impl should call mark_as_mutable_if_necessary:

sobjectizer/dev/so_5/message.hpp

Lines 862 to 876 in 5035f99

	template< bool is_signal, typename Msg >
	struct make_message_instance_impl
	{
	using E = typename message_payload_type< Msg >::envelope_type;
	template< typename... Args >
	[[nodiscard]]
	static std::unique_ptr< E >
	make( Args &&... args )
	{
	ensure_not_signal< Msg >();
	return std::unique_ptr< E >( new E( std::forward< Args >(args)... ) );
	}
	};

Without this call limit_then_transform can make a mutable message and send it into MPMC mbox.

[eao197]

The fix will be a part of upcoming 5.8.1 release.