When less than six places are given for microseconds, we zerofill from
the right (leftmost place is always 1/10s). We only did this when all
announced date/time fields were given; now we also format fractional
seconds when more significant fields are left out.


mysql-test/r/func_time.result:
  show that we treat fractions of seconds correctly (zerofill from
  right to six places) even if we left out fields on the left
mysql-test/t/func_time.test:
  show that we treat fractions of seconds correctly (zerofill from
  right to six places) even if we left out fields on the left
sql/item_timefunc.cc:
  format fractions of seconds even if announced
  more significant fields were left out