net/http: revert overly-strict part of earlier smuggling defense

net/http: revert overly-strict part of earlier smuggling defense

The recent https://golang.org/cl/11810 is reportedly a bit too
aggressive.

Apparently some HTTP requests in the wild do contain both a
Transfer-Encoding along with a bogus Content-Length. Instead of
returning a 400 Bad Request error, we should just ignore the
Content-Length like we did before.

Change-Id: I0001be90d09f8293a34f04691f608342875ff5c4
Reviewed-on: https://go-review.googlesource.com/11962
Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>