Fix path regex substitution

[Noel215]

Hi,

This commit broke the method process_file when the input_file_name argument if of type pathlib.Path instead of os.path (string).

This PR fixes that issue, adding a test for compatibility with both path types

Already reported in the following issues:

• ros2 version can crash in process_file if input_file_name is not of string type. #374
• Call to xacro.process_file in utils.py load_xacro can/will throw PickNikRobotics/launch_param_builder#16

[acalhoon]

Thanks for the quick patch and for creating the PR for me!

[rhaschke]

Thanks for the PR. Merged via 3d061ea and 46cb051.