Omit an extra CRLF at the end of message bodies.

For a message boundary 'x', the full delimiter should be '\r\n--x\r\n', where
the CRLFs are part of the delimiter and not part of the message on either side
of it. Unfortunately, kamailio was including the leading CRLF in messages when
filter_body was called.
Add an explanatory comment.
Merge pull request #3 from flowroute/fix-filter-body

[DEV-566] Omit an extra CRLF at the end of message bodies when calling filter_body