net/http: handle 3xx responses with no Location

net/http: handle 3xx responses with no Location

RFC 7231 does not require that a 3xx response contain a Location header.
When receiving such a response, just return it to the caller rather than
treating it as an error.

Fixes #49281.

Change-Id: I66c06d81b0922016384a0f4ff32bf52e3a3d5983
Reviewed-on: https://go-review.googlesource.com/c/go/+/375354
Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Trust: Brad Fitzpatrick <bradfitz@golang.org>