"BIT_AND() result in GROUP BY different when SQL_BIG_RESULT used"

return value of BIT_AND changed to BIGINT SIGNED
Also the patch fixes another bug: when temporary table is in use and one of
values in group is NULL, BIT_AND always returns zero.
Fixed it to always skip null values.


mysql-test/r/func_group.result:
  bug #1790: test results fixed
sql/item_sum.cc:
  fix for bug #1790:
  update_field() rewritten to use add() and thus was moved
  to Item_sum_bit::update_field()
  Item_sum_bit::reset_field() was rewritten to take into account
  reset_bits.
sql/item_sum.h:
  fix for bug #1790:
  Item_sum::update_field() and Item_sum::reset_field() commented
  Item_sum_and changed to return BIGINT SIGNED
  Item_sum_and::update_field() and Item_sum_or::update_field)
  were replaced with generic Item_sum_bit::update_field()