API: Return sparse objects always for cumsum
Always return `SparseArray` and `SparseSeries` for `SparseArray.cumsum()` and `SparseSeries.cumsum()` respectively, regardless of `fill_value`.

Close #12855.
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14771?src=pr) is 85.26% (diff: 100%)
> Merging [#14771](https://codecov.io/gh/pandas-dev/pandas/pull/14771?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #14771   diff @@
==========================================
  Files           144        144          
  Lines         50981      50983     +2   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43470      43472     +2   
  Misses         7511       7511          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [81a2f79...83314fc](https://codecov.io/gh/pandas-dev/pandas/compare/81a2f792a342a047bcc98b45cd6e99e6d82010b3...83314fc1d6f13bee0679aebef64abb7c4c63c258?src=pr) @jreback : I addressed all of the comments, and everything is passing.  Ready to merge if there are no other concerns. @jreback : Applied requested changes, and all is green.  Ready to merge if there are no other concerns. lgtm. thanks!