The buildfarm says commit 58274728fb doesn't
work so well on Windows. This is because the encoding part of Windows
locale names can be just a code page number, eg "1252", which we don't
consider to be a valid encoding name. Add a check to accept encoding
parts that are case-insensitively string equal; this at least ensures
that the new code doesn't reject any cases that the old code allowed.
cd3e0071b8