internal/export/idna: don't remove leading dots

Dots are still removed for profiles for which this makes sense.
Update golang/go#19821

Change-Id: I3de20bc5ddd943557831d1de998554105d7c07e7
Reviewed-on: https://go-review.googlesource.com/44380
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
2 files changed