ctype functions require their argument to be EOF or representable as an

ctype functions require their argument to be EOF or representable as an
unsigned char.  On platforms were char is signed, passing plain char won't
cut it.  Make sure we case chars to unsigned char before passing them to
tolower().

git-svn-id: svn://svn.tartarus.org/sgt/charset@8404 cda61777-01e9-0310-a592-d414129be87e