net/http: support multiple identical Content-Length headers

net/http: support multiple identical Content-Length headers

Referencing RFC 7230 Section 3.3.2, this CL
deduplicates multiple identical Content-Length headers
of a message or rejects the message as invalid if the
Content-Length values differ.

Fixes #16490

Change-Id: Ia6b0f58ec7d35710b11a36113d2bd9128f693f64
Reviewed-on: https://go-review.googlesource.com/31252
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>