BUG: Series.apply(to_timedelta) returns object-type Series.
_For later (post 0.13)..._

Good:

```
In [1]: Series(['2013-01-05']).apply(pd.to_datetime)
Out[1]:
0   2013-01-05 00:00:00
dtype: datetime64[ns]
```

Bad:

```
In [2]: Series(['00:00:01']).apply(pd.to_timedelta)
Out[2]:
0    1000000000 nanoseconds
dtype: object
```

This problem turned up in #5438 , resovling #5437 . Refer to the discussion there.

See `tseries/tests/test_timedeltas.py`, specifically `test_apply_to_timedelta`. Currently, Lines 304 and 311 of the latter contain commented out assert statements that, after this is resolved should be uncommented and pass.

Also see `tests/test_frame.py/test_operators_timedelta64.py`.
