net/http: handle Trailer headers according to RFC 7230

[urld]

The method declareTrailer in net/http/server.go does not handle Trailer headers according to RFC 7230.

While the the obsoleted RFC 2616 section 14.40 states:

Message header fields listed in the Trailer header field MUST NOT
include the following header fields:

• Transfer-Encoding
• Content-Length
• Trailer

declareTrailer only ignores the headers mentioned above, but RFC 7230 section 4.1.2 states:

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).

x/net/http2 already implements this behavior in ValidTrailerHeader. Maybe this method can be moved to net/http.

[ianlancetaylor]

CC @bradfitz @tombergan

[bradfitz]

We don't want to export this function in net/http, but we can move it somewhere common in x/net and vendor it in, perhaps under https://godoc.org/golang.org/x/net/http somewhere alongside httpproxy. Maybe httpimpl or httpguts or httpdetails.

[gopherbot]

Change https://golang.org/cl/104042 mentions this issue: http2: move ValidTrailerHeader to common package http/httpguts

[gopherbot]

Change https://golang.org/cl/104075 mentions this issue: net/http: omit forbidden Trailer headers from response

[gopherbot]

Change https://golang.org/cl/111655 mentions this issue: net/http: update bundled http2

[gopherbot]

Change https://golang.org/cl/111875 mentions this issue: lex/httplex, http/httpguts: merge the httplex package into httpguts

[gopherbot]

Change https://golang.org/cl/112996 mentions this issue: devapp/owners: remove net/lex