BUG: GH25495 incorrect dtype when using .loc to set Categorical value for column in 1-row DataFrame
- [x] closes https://github.com/pandas-dev/pandas/issues/25495
- [x] 1 test added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

