net/http: reject HTTP/1.1 Content-Length with sign in response

net/http: reject HTTP/1.1 Content-Length with sign in response

Enforces section 14.13 of RFC 2616 so that Content-Length header
values with a sign such as "+5" will be rejected.

Updates #39017

Change-Id: Icce9f00d03c8475fe704b33f9bed9089ff8802f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/234817
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>