[mod_sofia] Set the proper Content-Length for multipart messages

[winsock]

Context

This change ensures that the proper content length is set when setting a multipart header with custom headers.

Questions

• Is Content-Length even required or needed in the multipart data reading this email chain it seems like it may be unnecessary and unused but allowed since it isn't explicitly disallowed. Multipart handling can be simplified if we do not need it!
  • https://lists.cs.columbia.edu/pipermail/sip-implementors/2012-March/028216.html

Overview

• Use strstr to find the boundary of the body \r\n\r\n ignore if there is no valid body
• Use the pointer found by strstr + 4(accounting for the boundary) to call strlen with to get the real size of the body. But add one to match existing behavior.

[winsock]

Here is a packet capture of how this looks before and after:
before_and_after_pcap.zip