BUG: Negative RangeIndex can have positive step
Python allows positve steps in ranges, e.g.

```python
>>> rng = range(2, -2)
>>> rng.step
1  # positive step in a negative range
list(rng)
[]  # empty
```

which surprises me and surely must be a bug in Python. This affects pandas' RangeIndex:

```python
>>> idx = pd.RangeIndex(2, -2)
>>> df = pd.DataFrame(idx, index=idx)
>>> df.shape
(0, 1)  # 0 length
>>> df.index
RangeIndex(start=2, stop=-2, step=1)  # positive step!
>>> df  # diplays as empty, even though isn't empty
Empty DataFrame
Columns: [0]
Index: []
>>> df.sum()  # result in master, wrong
0    0
dtype: int64
>>> df.sum()  # Correct result, not in master
0    2
dtype: int64
```

We see that the dataframe displays as empty, even though it is not actually empty and aggregations give wrong results.

Do people agree that in cases of negative ranges, we should supply a default step of -1?