SIP Parser is not in compliance of RFC

[alexzubovsky]

https://github.com/davehorton/drachtio-sip/blob/75f5dac8d128ceb6bf04bf288f816a51c23fbdd9/lib/parser.js#L395

By https://www.w3.org/Protocols/rfc1341/7_2_Multipart.html, the boundary starts with \r\n-- and following \r\n\ belongs to the segment:

        delimiter := CRLF "--" boundary   ; taken from  Content-Type 
        field. 
                                       ;   when   content-type    is 
        multipart 
                                     ; There must be no space 
                                     ; between "--" and boundary.