The receiver itself is not transmitted and does not need to be
marshalable by encoding/gob.

Fixes #16803.

Change-Id: I42a3603fb7d3b36c97dcc2e51a398cd65ec3227d
Reviewed-on: https://go-review.googlesource.com/32094
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>