correct the bitmap_set_bit when a field is timestamp and described 
with default CURRENT_TIMESTAMP or on update CURRENT_TIMESTAMP,
then it will reduce a little time cost when the field doesnot need 
to write.


sql/sql_update.cc:
  here table->timestamp_field represent only the timestamp field related to CURRENT_TIMESTAMP,
  when the timestamp_field_type is 0 or 1, it need not to write the field data further.