BUG/API: Series arithmetic ops inconsistently hold names
Based on `Index` behavior, `Series` arithmetic should reset name when names are different?

```
import pandas as pd
s1 = pd.Series([1, 2, 3], name='a')
s2 = pd.Series([3, 4, 5], name='b')

(s1 + s2).name
# None

s1.add(s2).name
# a
```
