Alter handling for UNION syntax
  Tests for UNION and parentheses


mysql-test/r/union.result:
  Bug#2435
    Tests for UNION and parentheses
mysql-test/t/union.test:
  Bug#2435
    Tests for UNION and parentheses
sql/sql_yacc.yy:
  Bug#2435
    Amend handling of UNION with parentheses.