org-odt: Correctly export iso-8859-1 files with non-ascii chars

* contrib/lisp/org-odt.el (org-odt-get): Set
CODING-SYSTEM-FOR-WRITE and CODING-SYSTEM-FOR-SAVE to 'utf-8
irrespective of buffer-file-coding-system.

Fixes issue reported by Renzo Been in the following post.
http://lists.gnu.org/archive/html/emacs-orgmode/2011-07/msg00795.html

contrib/lisp/org-odt.el

@@ -1380,6 +1380,8 @@ MAY-INLINE-P allows inlining it as an image."
     (PLAIN-TEXT-MAP '(("&" . "&amp;") ("<" . "&lt;") (">" . "&gt;")))
     (TABLE-FIRST-COLUMN-AS-LABELS nil)
     (FOOTNOTE-SEPARATOR (org-lparse-format 'FONTIFY "," 'superscript))
+    (CODING-SYSTEM-FOR-WRITE 'utf-8)
+    (CODING-SYSTEM-FOR-SAVE 'utf-8)
     (t (error "Unknown property: %s"  what))))
 
 (defun org-odt-parse-label (label)