net/http: do not send Transfer-Encoding: identity in responses

Server handlers may set a "Transfer-Encoding: identity" header on
responses to disable chunking, but this header should not be sent
on the wire.

Fixes #49194.

Change-Id: I46a9e3b8ff9d93edd7d1c34d264fc309fa322ad5
Reviewed-on: https://go-review.googlesource.com/c/go/+/359176


Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>