Do a case-insensitive search for Content-Type.
Correct kamailio's filter_body function.

This time, kamailio wasn't correctly handling multipart MIME messages where the
content-type header within the MIME part came before other headers, since
kamailio expects that content-type will be the last and/or only header.

Specifically, messages that looked like this:

  --boundary
  Content-Type: application/sdp
  Content-Description: Description_Sdp_Message_Body
  Content-Disposition: session

The filtered body would still include the Content-Description and
Content-Disposition headers, despite that they aren't part of the message body.
Merge pull request #4 from flowroute/fix-filter-body

Fix filter_body (again)