temporary table BLOB now is longblob


mysql-test/r/create.result:
  blob size changed for safety
mysql-test/r/type_blob.result:
  blob size changed for safety
mysql-test/r/type_ranges.result:
  blob size changed for safety
mysql-test/r/union.result:
  blob size changed for safety
  new tests of UNION types merging
mysql-test/t/union.test:
  new tests of UNION types merging
sql/field.h:
  blob size changed for safety
sql/item.cc:
  processing of collation added
  added comment
sql/item.h:
  joining of UNION fields may failed now, because of incompatibility of collations
sql/sql_union.cc:
  joining of UNION fields may failed now, because of incompatibility of collations