[Minor] Support INSHEADER in the milter API

src/libserver/milter.c

@@ -1119,6 +1119,7 @@ rspamd_milter_send_action (struct rspamd_milter_session *session,
 		memcpy (pos, value->str, value->len + 1);
 		break;
 	case RSPAMD_MILTER_CHGHEADER:
+	case RSPAMD_MILTER_INSHEADER:
 		idx = htonl (va_arg (ap, guint32));
 		name = va_arg (ap, GString *);
 		value = va_arg (ap, GString *);

src/libserver/milter.h

@@ -31,6 +31,7 @@ enum rspamd_milter_reply {
 	RSPAMD_MILTER_CHGFROM = 'e',
 	RSPAMD_MILTER_ADDHEADER = 'h',
 	RSPAMD_MILTER_CHGHEADER = 'm',
+	RSPAMD_MILTER_INSHEADER = 'i',
 	RSPAMD_MILTER_REJECT = 'r',
 	RSPAMD_MILTER_TEMPFAIL = 't',
 	RSPAMD_MILTER_REPLYCODE = 'y',