This bug manifested itself for queries with grouping by columns of
the BIT type. It led to wrong comparisons of bit-field values and
wrong result sets.
Bit-field values never cannot be compared as binary values. Yet
the class Field_bit had an implementation of the cmp method that
compared bit-fields values as binary values. 
Also the get_image and set_image methods of the base class Field 
cannot be used for objects of the Field_bit class. 
Now these methods are declared as virtual and specific implementations
of the methods are provided for the class Field_bit.