* message.el (message-fix-before-sending): Don't expose invisible MIME parts;

* message.el (message-fix-before-sending): Don't expose invisible MIME parts;
 don't use `message-find-invisible-regions'.
(message-find-invisible-regions): Remove.
(invisible-region): Advise the function to add the text property
 `mime-edit-invisible'.
(message-invisible-region): Remove.
(message-save-drafts): Copy all the text properties from the editing buffer to
 the encoding buffer.
(message-send): Ditto.