fixed problem with long string results of expressions in UNIONS (BUG#10025)

mysql-test/r/union.result:
  test for Bug #10025
mysql-test/t/union.test:
  test for Bug #10025
sql/item.cc:
  set HA_OPTION_PACK_RECORD and change type to MYSQL_TYPE_STRING, to allow correct field creation in case of length more then 255 (creation STRING field with length more then 255)

mysql-test/r/union.result

@@ -1235,3 +1235,17 @@ show columns from t2;
 Field	Type	Null	Key	Default	Extra
 a	varchar(3)	YES		NULL	
 drop table t2, t1;
+CREATE TABLE t1 (a mediumtext);
+CREATE TABLE t2 (b varchar(20));
+INSERT INTO t1 VALUES ('a'),('b');
+SELECT left(a,100000000) FROM t1 UNION  SELECT b FROM t2;
+left(a,100000000)
+a
+b
+create table t3 SELECT left(a,100000000) FROM t1 UNION  SELECT b FROM t2;
+show create table t3;
+Table	Create Table
+t3	CREATE TABLE `t3` (
+  `left(a,100000000)` longtext
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+drop tables t1,t2,t3;

mysql-test/t/union.test

@@ -742,3 +742,14 @@ create table t2 select a from t1 union select c from t1;
 create table t2 select a from t1 union select b from t1;
 show columns from t2;
 drop table t2, t1;
+
+#
+# correct conversion long string to TEXT (BUG#10025)
+#
+CREATE TABLE t1 (a mediumtext);
+CREATE TABLE t2 (b varchar(20));
+INSERT INTO t1 VALUES ('a'),('b');
+SELECT left(a,100000000) FROM t1 UNION  SELECT b FROM t2;
+create table t3 SELECT left(a,100000000) FROM t1 UNION  SELECT b FROM t2;
+show create table t3;
+drop tables t1,t2,t3;

sql/item.cc

@@ -3099,8 +3099,8 @@ Field *Item_type_holder::make_field_by_type(TABLE *table)
                          enum_set_typelib, collation.collation);
   case MYSQL_TYPE_VAR_STRING:
     table->db_create_options|= HA_OPTION_PACK_RECORD;
-    return new Field_string(max_length, maybe_null, name, table,
-                            collation.collation);
+    fld_type= MYSQL_TYPE_STRING;
+    break;
   default:
     break;
   }