mysql-test/r/union.result:
  Test cases for bugs #2508 and #2552
mysql-test/t/union.test:
  Test cases for bugs #2508 and #2552
sql/sql_class.h:
  A fix for a bug #2508
sql/sql_select.cc:
  A fix for a bug #2508
sql/sql_union.cc:
  a fix for a bug #2552