Update "sent-by" field of Via header on CANCEL request for proxy scenario

[riza]

From ​rfc3261:

For each target, the proxy forwards the request following these
   steps:
      1.  Make a copy of the received request
      2.  Update the Request-URI
      3.  Update the Max-Forwards header field
      4.  Optionally add a Record-route header field value
      5.  Optionally add additional header fields
      6.  Postprocess routing information
      7.  Determine the next-hop address, port, and transport
      8.  Add a Via header field value
      9.  Add a Content-Length header field if necessary
      10. Forward the new request
      11. Set timer C

On proxy scenario, Via header required to be added to the message sent.
The library will add the header on sip_util_proxy.c without the sent-by field which is updated on sip_util.c.

Unfortunately, #2085 will not update Via header on CANCEL request making the Via header without the sent-by field. Afterwards the message will get ignored/discarded by the receiving end.

This ticket will update the Via header when the sent-by field is not set.

Thanks to Arun Ahluwalia for the report