BUG: groupby cumsum with axis=1 computes cumprod
- [x] closes #13993
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

lgtm. ping on green.
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13994?src=pr) is 85.28% (diff: 100%)

> Merging [#13994](https://codecov.io/gh/pydata/pandas/pull/13994?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13994   diff @@
==========================================
  Files           139        139          
  Lines         50206      50206          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42817      42820     +3   
+ Misses         7389       7386     -3   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [6f88416...56dc1c7](https://codecov.io/gh/pydata/pandas/compare/6f8841648d068cc04c7bfe0625f131b7c7d6469e...56dc1c759c4ffc2f14543991e9ac0e7896241150?src=pr)
 @jreback ~~All green.~~ I swear, it was.
 green means travis is green (its yellow atm).
 @jreback OK, _now_ it's all green.
 thanks!
