bug tr remove some characters when used with -s

[Saïvann]

Hi

I used tr to filter locale names in a debian package but I got a problem
with a specific field. When using tr with "-s", it removes a "n" in
"nn-NO". You can reproduce this bug simply with this command in a terminal :

echo "nn-NO" | tr -s A-Z a-z

The bug is not reproducible with this command :

echo "nn-NO" | tr A-Z a-z

I don't know if that is intended but it looks like a bug. I'm currently
using coreutils 6.10-6ubuntu1 on ubuntu.

Thanks for your incredible work!

-- 
Saïvann