BUG: reset_index can reset index name
```python
import pandas as pd
assert (pd.DataFrame(index=pd.Index([], name='x'))
        .reset_index(level=[]).index.name) is None
```

The root of the problem is in `pandas.core.common._default_index`:
```python
def _default_index(n):
    from pandas.core.index import RangeIndex
    return RangeIndex(0, n, name=None)
```

Probably, a new optional argument `name` can be introduced to `_default_index` and used in `reset_index`.