Converting BIT to a string (an intermediate step in conversion) does 
not yield an ASCII numeric string, so we skip that step for BIT and
get the integer value directly from the item.

This site in sql/item_strfunc.cc may be ripe for refactoring for
other types as well, where converting to a string is a waste of time.


mysql-test/r/type_bit.result:
  Test that conversion functions on BIT types work properly, including 
  NULL.
mysql-test/t/type_bit.test:
  Test that conversion functions on BIT types work properly.
sql/item_strfunc.cc:
  BIT is unlike the other numeric types, in that when we convert it
  to a String, it becomes a one-byte string with ordinal numeric value
  of the BIT field, not a several-byte string with the ASCII decimal
  representation.  As a special case for conversion functions, we take
  the integer directly from the bit type instead of representing it
  as a string in an intermediate step.