It seems this one got overlooked in the switch to the "trailers" T-E in
draft-ietf-http-v11-spec-rev-05:
Section 14.40 TE:
A server MUST NOT include any header fields unless the "chunked"
transfer-coding is present in the request as an accepted transfer-
coding in the TE field.
Replace "chunked" by "trailers".
Cheers,
Ronald