a temporary table.

The result string of the Item_func_group_concat wasn't initialized in the 
copying constructor of the Item_func_group_concat class. This led to a
wrong charset of GROUP_CONCAT result when the select employs a temporary
table.

The copying constructor of the Item_func_group_concat class now correctly
initializes the charset of the result string.


mysql-test/t/func_gconcat.test:
  Added a test case for the bug#29850: Wrong charset of the GROUP_CONCAT result
  when the select employs a temporary table.
mysql-test/r/func_gconcat.result:
  Added a test case for the bug#29850: Wrong charset of the GROUP_CONCAT result
  when the select employs a temporary table.
sql/item_sum.cc:
  Bug#29850: Wrong charset of GROUP_CONCAT result when the select employs
  a temporary table.
  The copying constructor of the Item_func_group_concat class now correctly
  initializes the charset of the result string.