Unexpected names set when special characters are used

[oleksiyskononenko]

>>> from datatable import dt
>>> DT = dt.Frame({"\t" : [1]})
>>> DT.names
('.',)

[oleksiyskononenko]

Seems to be a restriction by design: https://datatable.readthedocs.io/en/latest/manual/name_mangling.html#name-mangling