`.replace()` doesn't work for `time64` columns

[oleksiyskononenko]

It seems that .replace() doesn't have any effect when replacing times:

from datatable import dt
from datetime import datetime as d

DT = dt.Frame([d(2000, 1, 1, 1, 1)])
DT.replace({d(2000, 1, 1, 1, 1) : d(2222, 1, 1, 1, 1)})
print(DT)

produces

   | C0                 
   | time64             
-- + -------------------
 0 | 2000-01-01T01:01:00
[1 row x 1 column]

should be

   | C0                 
   | time64             
-- + -------------------
 0 | 2222-01-01T01:01:00
[1 row x 1 column]