API/BUG: Index creation with named Index resets name
```
import pandas as pd
idx = pd.Index([1, 2 ,3], name='x')
pd.Index(idx)
# Int64Index([1, 2, 3], dtype='int64')
```

A point is whether to reset existing Index name in below case. Current impl can't distinguish whether name is not passed, or passed `None` explicitly.

```
pd.Index(idx, name=None)
# -> should reset name?
```
