- put the code that sets HA_NULL_PART bit back
- Fix test_if_ref/part_of_refkey() so that 
  = NULL-ability of lookup columns does not prevent the equality 
    from being removed (we now have early/late NULLs filtering which 
    will filter out NULL values)
  = equality is not removed if it is ref_or_null access, and the value 
    of the lookup column can alternate between the lookup value and NULL.