net/http: further restrict which trailers may be sent

[bradfitz]

http://tools.ietf.org/html/rfc7230#section-4.1.2

   A sender MUST NOT generate a trailer that contains a field necessary
   for message framing (e.g., Transfer-Encoding and Content-Length),
   routing (e.g., Host), request modifiers (e.g., controls and
   conditionals in Section 5 of [RFC7231]), authentication (e.g., see
   [RFC7235] and [RFC6265]), response control data (e.g., see Section
   7.1 of [RFC7231]), or determining how to process the payload (e.g.,
   Content-Encoding, Content-Type, Content-Range, and Trailer).

[gopherbot]

CL https://golang.org/cl/23234 mentions this issue.