Follow the convertion (https://golang.org/s/generatedcode) for generated
code in stringer.go.

Change-Id: I7b5fbb04ba03e8ac77a9a0a402088669469de858
Reviewed-on: https://go-review.googlesource.com/122015
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>