net/http/httputil: close incoming ReverseProxy request body

Reading from an incoming request body after the request handler aborts
with a panic can cause a panic, becuse http.Server does not (contrary
to its documentation) close the request body in this case.

Always close the incoming request body in ReverseProxy.ServeHTTP to
ensure that any in-flight outgoing requests using the body do not
read from it.

Updates #46866
Fixes CVE-2021-36221

Change-Id: I310df269200ad8732c5d9f1a2b00de68725831df
Reviewed-on: https://go-review.googlesource.com/c/go/+/333191


Trust: Damien Neil <dneil@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>