bit has special syntax for storing and retreiving so we'd need a
converter for that.
Storing 1 bit will result in one byte left padded on the disc so the
result of a query for boolean 0 will be 0x00 and for boolean 1 will be
0x80.

this allows different implementation in different DBMS

CUBRID does not supprt PDO::TYPE_BOOL, so we use STRING here which will
be casted by the DBMS

type is array in PHP documentation

commit in redis branch was:
0cd65e74