t4041, t4205, t6006, t7102: use iso8859-1 rather than iso-8859-1

Both "iso8859-1" and "iso-8859-1" are understood as latin-1 by
modern platforms, but the latter is not understood by older
platforms;update tests to use the former.

This is in line with 3994e8a9 (t4201: use ISO8859-1 rather than
ISO-8859-1, 2009-12-03), which did the same.

Signed-off-by: Alexey Shumkin <Alex.Crezoff@gmail.com>
Reviewed-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>