fix: end a record with \n\r

[rianby64]

Text fields such as the session name and information are octet strings that may contain any octet with the exceptions of 0x00 (Nul), 0x0a (ASCII newline), and 0x0d (ASCII carriage return). The sequence CRLF (0x0d0a) is used to end a record, although parsers SHOULD be tolerant and also accept records terminated with a single newline character.

[rianby64]

emiago/media#1 (comment)

I played with other soft-phones and they accept that the end of the SDP ends without \n\r:

• gnome-calls
• zoiper
• microsip
• baresip - doesn't accept SDP unless you add \n\r at the end of the SDP response

I saw the results from other SDP implementations and they add \n\r at the end of the answer.
This issue looks more like the enforcing of the good practice of having at the end of the file an empty line, like in git 😄 so it's up to you if this makes sense or not.

In any case, I'd like to play with baresip and have fun... it's just one empty line 😃 🙈