You are working in a small Python project in the current directory. The file `string_utils.py` defines an `is_palindrome` function, but several tests in `test_string_utils.py` currently fail.

Your task: fix `is_palindrome` in `string_utils.py` so that ALL tests in `test_string_utils.py` pass when run with `pytest`. Run `pytest` yourself to verify. Do NOT modify the test file.
