Compliance: rename Trailers header to Trailer everywhere.
RFC 2616 section 13.5.1 has a typo in Trailers header name. The correct name is Trailer. See http://trac.tools.ietf.org/wg/httpbis/trac/ticket/9
Co-Advisor test cases: test_case/rfc2616/hopHdr-Trailer-toClt
test_case/rfc2616/hopHdr-Trailer-toSrv
