BUG: inconsistency in some timedelta64[ns] reduction operations
```
In [1]: s = pd.to_timedelta(range(4),unit='d')

In [2]: s
Out[2]: 
0   0 days
1   1 days
2   2 days
3   3 days
dtype: timedelta64[ns]

In [3]: s.mean()
Out[3]: 
0   1 days, 12:00:00
dtype: timedelta64[ns]

In [4]: s.sum()
Out[4]: 518400000000000
```
