t3900: use ancient iconv names for backward compatibility

t3900: use ancient iconv names for backward compatibility

Some old iconv implementations do not have many alternate names and/or
do not match character encoding names case insensitively.  These
implementations can not tell that utf-8 and UTF-8 are the same encoding
and fail when trying to do the conversion.  So use the old names, which
modern implementations still support.

The following conversions were performed:

         utf-8 --> UTF-8
    ISO-8859-1 --> ISO8859-1
         EUCJP --> eucJP

Also update t9129 and t9500 which make use of the test files in t/t3900.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>