fix cancel and proxy example

[yeoleobun]

• Remove state transition in send_cancel
  Since CANCEL and INVITE are separated transactions, in transaction of INVITE should be COMPLETE by final response of INVITE, like 200 or 487. If transaction completed here, it will prevent it receiving final response like 487 and ACK to it.
• update to tag of placeholder ACK to 487, fix the dropped INVITE ACK, have no to tag
• Fix proxy example, use same transaction id in Via header for CANCEL and INVITE