BUG: Accept kwargs kind and na_position in Series.sort_index (GH13589)
- [x] closes #13589 
- [x] tests added / passed
- [x] whatsnew entry

/cc @jreback 

you always needs tests 
 @jreback I'm not sure what to add in tests for this one. Can you please help me here?
 you are adding keywords so need tests - look at what Dataframe tests for snd mirror those 
 I tried looking into `test_frame.py`. It has no test containing `kind` or `na_position`. Am I missing something or looking at wrong place?
 rebase on master you have a really old master 
just recursive grep
 > just recursive grep

Thanks for the hint, @jreback. Please check the tests I have added if they are sufficient or not.
 @jreback Is this fine?
 @sahildua2305 Only a few small comments, for the rest this is already looking fine!
 @jorisvandenbossche I've updated the PR. Please check if it's good now.
 can you rebase / update?
 Yes! I'll do so. 
 closing in favor of #14445 