Added test case for bug#18759 Incorrect string to numeric conversion.  
select.test:
  Added test case for bug#18759 Incorrect string to numeric conversion.
item_cmpfunc.cc:
  Cleanup after fix for bug#18360 removal


sql/item_cmpfunc.cc:
  Cleanup after fix for bug#18360 removal
mysql-test/t/select.test:
  Added test case for bug#18759 Incorrect string to numeric conversion.
mysql-test/r/select.result:
  Added test case for bug#18759 Incorrect string to numeric conversion.