BUG: index name lost with timedelta ops
```
import pandas as pd 

dtidx = pd.DatetimeIndex(['2011-01-01'], freq='D', name='dtidx')
(dtidx + 1).name
# dtidx

# NG
(dtidx + pd.Timedelta('1 day')).name
# None

tdidx = pd.TimedeltaIndex(['1 day'], freq='D', name='tdidx')
(tdidx + 1).name
# tdidx

# NG
(tdidx + pd.Timedelta('1 day')).name
# None
```

ref: #9862
