sort --ignore-case option changes underscore sort position

[jrw32982]

I couldn't find this previously reported.  I didn't see anything in
the documentation which indicates that this is expected behavior.

To replicate the bug:

$ sort --version
sort (coreutils) 5.2.1
...
$ export LC_ALL=C
$ { echo a_; echo ax; } | sort
a_
ax
$ { echo a_; echo ax; } | sort --ignore-case
ax
a_

Thanks!
-- John Wiersba