BUG:#29928 Fix to_json output 'table' orient for single level MultiIndex.
`dataframe.to_json()` was writing incorrect index field name, so applying read_json resulted in NaN index values. 
`dataframe.to_json()` now converts single level MultiIndex into single Index before encoding.


- [x] closes #29928 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

is there an associated issue? > 
> 
> is there an associated issue?

Yes, #29928 Considering I have no news on this PR, is it suitable, or is there anything I can do to improve it ? @LucasG0 can you fix merge conflicts and move note to 1.2? Someone will take a look again after that @WillAyd Done, I can restore test for timedeltas if needed.  lgtm @jreback  @jreback maybe you have an opinion on this ? :) Thanks I will check it. This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. closing as stale. if you want to continue, pls ping and can re-open.