In case of ROW item each compared pair does not
check if argumet collations can be aggregated and
thus appropiriate item conversion does not happen.
The fix is to add the check and convertion for ROW
pairs.


mysql-test/r/row.result:
  test result
mysql-test/t/row.test:
  test case
sql/item.cc:
  added agg_item_set_converter() function which was a part of
  agg_item_charsets() func. The only difference is that
  agg_item_set_converter() checks and converts items 
  using already known collation.
sql/item.h:
  added agg_item_set_converter() function
sql/item_cmpfunc.cc:
  In case of ROW item each compared pair does not
  check if argumet collations can be aggregated and
  thus appropiriate item conversion does not happen.
  The fix is to add the check and convertion for ROW
  pairs.