Two missing methods added to Item_splocal + test case.


mysql-test/r/sp.result:
  Test case for BUG#1547
mysql-test/t/sp.test:
  Test case for BUG#1547
sql/item.h:
  Fix for BUG#1547: "SELECT ... WHERE field = var" sometimes give the wrong result in SPs
  
  Added missing methods to Item_splocal to get the correct compare function
  and make the optimizer do the right thing.