used

In a simple queries a result of the GROUP_CONCAT() function was always of 
varchar type.
But if length of GROUP_CONCAT() result is greater than 512 chars and temporary
table is used during select then the result is converted to blob, due to
policy to not to store fields longer than 512 chars in tmp table as varchar
fields.

In order to provide consistent behaviour, result of GROUP_CONCAT() now
will always be converted to blob if it is longer than 512 chars.
Item_func_group_concat::field_type() is modified accordingly.


mysql-test/t/func_gconcat.test:
  Added test case for bug#14169: type of group_concat() result changed to blob if tmp_table was used
mysql-test/r/func_gconcat.result:
  Added test case for bug#14169: type of group_concat() result changed to blob if tmp_table was used
sql/unireg.h:
  Added the CONVERT_IF_BIGGER_TO_BLOB constant
sql/sql_select.cc:
  Fixed bug#14169: type of group_concat() result changed to blob if tmp_table was used
  The unnamed constant 255 in the create_tmp_field() and create_tmp_field_from_item() functions now defined as the CONVERT_IF_BIGGER_TO_BLOB constant.
  The create_tmp_field() function now converts the Item_sum string result to a blob field based on its char length.
sql/item_sum.h:
  Fixed bug#14169: type of group_concat() result changed to blob if tmp_table was used
  To the Item_func_group_concat calls added the member function field_type() which returns the BLOB or VAR_STRING type based on the items length.
sql/item_func.cc:
  Fixed bug#14169: type of group_concat() result changed to blob if tmp_table was used
  In the Item_func::tmp_table_field() function the unnamed constant 255 is changed to the CONVERT_IF_BIGGER_TO_BLOB constant.
  The Item_func::tmp_table_field() function now measures the result length in chars rather than bytes when converting string result to a blob.