Per private report.

This isn't actually in the spec, but there's also no need to be so
permissive here.

Change-Id: I2b464778cc502ca7a99ea533622afea8f943eb7d
Reviewed-on: https://go-review.googlesource.com/124735
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>