Series[timedelta64] +/- TimedeltaIndex inherits name from TimedeltaIndex
Opening mainly for an issue to reference in upcoming PR.

This behavior is not present in 0.22.0, is present in master as of 04beec77e5a713dd

```
tdi = pd.TimedeltaIndex(['1day', '2days', '3days'])
ser = pd.Series(tdi)
tdi.name = 'foo'
ser.name = 'bar'

>>> (tdi + ser).name
'foo'
>>> (ser + tdi).name
'foo'
>>> (ser - tdi).name
'foo'
>>> (tdi - ser).name
'foo'
```

These names should all be coming back as `None`.