BUG: RangeIndex accepting length-1 arrays as argument
- [x] closes #13542
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13765?src=pr) is 85.27% (diff: 100%)

> Merging [#13765](https://codecov.io/gh/pydata/pandas/pull/13765?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #13765   diff @@
==========================================
  Files           139        139          
  Lines         50211      50214     +3   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42819      42820     +1   
- Misses         7392       7394     +2   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [5f49638...6180487](https://codecov.io/gh/pydata/pandas/compare/5f496380beb81d3986ca48cf41b893d95c227895...618048708ce4146b0e0a28ff605d84f94a875139?src=pr)
 @jreback OK?
 can you add the orginal issue example somewhere (and assert it works)
 That's what I added in `test_concat.py` (next to the addition in the tests in `test_range.py`) I think?
 is it a simplified example?

the original one is on a panel
 Well, looking back at the original issue https://github.com/pydata/pandas/issues/13542, this is indeed not the original example I included in the tests. But I found the reported example rather convoluted/difficult to comprehend what was going on. And the example I added, is a simplified (whithout using panels) example that experience exactly the same issue (it is what happened inside the groupby).

But I can add the original panel example as well if you like
 it's fine
a simplified example is often better anyhow
so lgtm
