The unsigned flag in Item was not propagated through the single value subqueries.
This caused the result to be treated as signed.


mysql-test/r/subselect.result:
  Added testcases for bug #19700
mysql-test/t/subselect.test:
  Added testcases for bug #19700
sql/item.h:
  Preserved the unsigned flag in Item_subselect
sql/item_subselect.cc:
  Preserved the unsigned flag in Item_subselect