core: skip to-tag when computing via branch for invite, ack and cancel

- proper functionality for stateless proxy in case of negative ACK (in
this case the INVITE has no To-tag, but ACK has it and resulted in
different via branch, making downstream proxy to not match transaction)

src/core/char_msg_val.h

@@ -38,6 +38,7 @@
 inline static int char_msg_val( struct sip_msg *msg, char *cv )
 {
 	str src[8];
+	str sempty = str_init("");
 
 	if (unlikely(!check_transaction_quadruple(msg))) {
 		LM_ERR("can't calculate char_value due to a parsing error\n");
@@ -55,7 +56,12 @@ inline static int char_msg_val( struct sip_msg *msg, char *cv )
 	}
 	/* use only the from & to tags */
 	src[0]=get_from(msg)->tag_value;
-	src[1]=get_to(msg)->tag_value;
+	if(msg->first_line.u.request.method_value
+			& (METHOD_INVITE|METHOD_ACK|METHOD_CANCEL)) {
+		src[1]=sempty;
+	} else {
+		src[1]=get_to(msg)->tag_value;
+	}
 	src[2]= msg->callid->body;
 	src[3]= msg->first_line.u.request.uri;
 	src[4]= get_cseq( msg )->number;