The 255-octet limit applies to wire format, not presentation format.

Fixes #17549

Change-Id: I2b5181c53fba32fea60178e0d8df9114aa992b55
Reviewed-on: https://go-review.googlesource.com/31722
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>