BUG: Fixed DataFrame.describe percentiles are ndarray w/ no median
Explicit conversion to list for `percentiles`. Fixes the case where `percentiles` is passed as a numpy with no median (0.5) present. Closes #14908.

Thx, can u add tests and whatsnew? ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14914?src=pr) is 84.64% (diff: 100%)
> Merging [#14914](https://codecov.io/gh/pandas-dev/pandas/pull/14914?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **0.07%**

```diff
@@             master     #14914   diff @@
==========================================
  Files           144        144          
  Lines         51057      51016    -41   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43180      43185     +5   
+ Misses         7877       7831    -46   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [f1cfe5b...5c8199b](https://codecov.io/gh/pandas-dev/pandas/compare/f1cfe5b4eb614f9b4e201e764e9bd25f2389b355...5c8199b89490c26e980c473219b1c4b75d4cf057?src=pr) small changes. ping on green. lgtm. ping on green. thanks!