Fix default site map

http://example.com/index.php?option=com_xmap&view=xml
or (SEF)
http://example.com/component/xmap/xml

The default site map was not showing manual edits made to the
"Change Freq." and "Priority" for individual items.  The
correct row from the #__xmap_items table was not being
retrieved.
Merge pull request #11 from zjw/default_site_map

Fix default site map