Bug 23506: Display correct icon in staff client and OPAC

Bug 23506: Display correct icon in staff client and OPAC

When the leader6 is set to 'i' the MARC21slim2intranetDetail.xsl
attempts to display 'SO.png' which does not exist.

TEST PLAN:
1. Find or create a record with an 'i' as the 6th position in the 000 leader.
2. Do to the OPAC detail page and staff detail for an item in that record.
3. Notice the material type icon does not display because SO.png does not exist.
4. Apply patch and restart_all
5. Look at the opac detail and staff detail page again after clearning the browser cache.
6. The material type icon now displays (MU.png), this is the same icon used on the OPAC results page.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>