when calculating GROUP_CONCAT all blob fields are transformed
  to varchar when making the temp table.
  However a varchar has at max 2 bytes for length. 
  This fix makes the conversion only for blobs whose max length 
  is below that limit. 
  Otherwise blob field is created by make_string_field() call.


mysql-test/r/func_gconcat.result:
  Bug#16712: group_concat returns odd srting insead of intended result
    * testsuite for the bug
mysql-test/t/func_gconcat.test:
  Bug#16712: group_concat returns odd srting insead of intended result
    * testsuite for the bug
sql/item_sum.cc:
  Bug#16712: group_concat returns odd srting insead of intended result
    * force blob->varchar conversion for small enough blobs only
sql/sql_select.cc:
  Bug#16712: group_concat returns odd srting insead of intended result
    * force blob->varchar conversion for small enough blobs only