TST: DataFrame.quantile should have Float64Index
The result of `DataFrame.quantile` should have `Float64Index` as below, but current test case doesn't check it.

```
import pandas as pd
pd.DataFrame([1, 2, 3, 4]).quantile([0.5, 0.75]).index
# Float64Index([0.5, 0.75], dtype='float64')
```
