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
