fix: end a record with \n\r

[rianby64]

As per RFC 4566, 5. SDP Specification:

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]

Hello @emiago !

I posted this pull-request as I'm playing with baresip and I found strange that baresip rejects the call because of the lack of \n\r. I checked in other places and found that the SDP message usually ends with \n\r.

[emiago]

Hi @rianby64, I guess it is not strict everywhere as Content Lenght also finishes last line.
On other hand I see no issue adding this if it is per RFC.
Have you played with other services?