BUG: to_datetime with Index arg does not retain name in all cases
```
#Example

In [17]: idx = pd.Index([15e9], name='name')

In [18]: pd.to_datetime(idx, errors='ignore', box=True, unit='s').name

In [19]: pd.__version__
Out[19]: '0.24.0.dev0+193.gdc45fba'
```
There are certain cases where the name attribute of the passed `Index` are not retained after `to_datetime`.

The `name` kwarg looks to be missing from these locations:

https://github.com/pandas-dev/pandas/blob/dc45fbafef172e357cb5decdeab22de67160f5b7/pandas/core/tools/datetimes.py#L102

https://github.com/pandas-dev/pandas/blob/dc45fbafef172e357cb5decdeab22de67160f5b7/pandas/core/tools/datetimes.py#L139

https://github.com/pandas-dev/pandas/blob/dc45fbafef172e357cb5decdeab22de67160f5b7/pandas/core/tools/datetimes.py#L344 
