Change mail header to 'MAIL'

See https://lcm.mi.infn.it/gogs/lcmstaff/lcmlog-server/pulls/2/ for info.

lcmlog

@@ -292,8 +292,8 @@ method_post()
 		read TAGS
 	done
 
-	SEND_MAIL="No"
-	ask "Send the log via email? [default: y]" "y" && SEND_MAIL="Yes"
+	SEND_MAIL="NOMAIL"
+	ask "Send the log via email? [default: y]" "y" && SEND_MAIL="MAIL"
 
 	# Update of the pipe
 	printf "%s\n" "POST" >> $pipe