message: sanitize maildir

Remove trailing '/' in maildirs, since people have that (like "/foo/"),
and earlier version didn't complain about that.

Fixes #2298

lib/message/mu-message.hh

@@ -208,6 +208,16 @@ public:
 	 */
 	Result<void> set_maildir(const std::string& maildir);
 
+	/**
+	 * Clean up the maildir. This is for internal use, but exposed for testing.
+	 * For now cleaned-up means "stray trailing / removed".
+	 *
+	 * @param maildir some maildir
+	 *
+	 * @return a cleaned-up version
+	 */
+	static std::string sanitize_maildir(const std::string& maildir);
+
 	/**
 	 * Get the subject of this message
 	 *