header: remove trailing colon in header field

With this commit trailing colons in the :header command's field are
removed to prevent double-colon-headers from being sent.

Signed-off-by: Moritz Poldrack <git@moritz.sh>
Acked-by: Robin Jarry <robin@jarry.cc>

commands/compose/header.go

@@ -56,6 +56,8 @@ func (Header) Execute(aerc *widgets.Aerc, args []string) error {
 
 	composer, _ := aerc.SelectedTab().(*widgets.Composer)
 
+	args[optind] = strings.TrimRight(args[optind], ":")
+
 	value := strings.Join(args[optind+1:], " ")
 
 	if !force {