BUG: fix explore() with highlight_kwds containing custom style_function

[BartoszBiernacki]

Fixes #3031

[jorisvandenbossche]

@BartoszBiernacki Thanks a lot for the PR, and our apologies for the slow follow-up.

Could you add a test for this? (check geopandas/tests/test_explore.py)

You also have a linter failure because of some trailing whitespace. To avoid this kind of issues, I would personally recommend installing the pre-commit hook we have included in the repo which will check and fix those lint issues automatically. See https://geopandas.org/en/latest/community/contributing.html#style-guide-linting