net/http: don't panic after request if Handler sets Request.Body to nil

The Server's server goroutine was panicing (but recovering) when
cleaning up after handling a request. It was pretty harmless (it just
closed that one connection and didn't kill the whole process) but it
was distracting.

Updates #13135

Change-Id: I2a0ce9e8b52c8d364e3f4ce245e05c6f8d62df14
Reviewed-on: https://go-review.googlesource.com/16572


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>