API: Should Index.min and max use nanmin and nanmax?
Index and Series `min` and `max` handles `nan` and `NaT` differently. Even though `min` and `max` are defined in `IndexOpsMixin`, `Series` doesn't use them and use `NDFrame` definitions.

```
pd.Index([np.nan, 1.0]).min()
# nan

pd.Index([np.nan, 1.0]).max()
# nan

pd.DatetimeIndex([pd.NaT, '2011-01-01']).min()
# NaT

pd.DatetimeIndex([pd.NaT, '2011-01-01']).max()
#2011-01-01 00:00:00

# Series excludes nan and NaT
pd.Series([np.nan, 1.0]).min()
#1.0

pd.Series([np.nan, 1.0]).max()
#1.0

pd.Series([pd.NaT, pd.Timestamp('2011-01-01')]).min()
#2011-01-01 00:00:00

pd.Series([pd.NaT, pd.Timestamp('2011-01-01')]).max()
#2011-01-01 00:00:00
```
