BUG: DataFrame.shift with axis=1 and mismatched fill_value
- [ ] closes #xxxx
- [x] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

user facing? > user facing?

yes, there's a whatsnew (forgot to check the checkbox until just now)