PosixPath instances should be seen as paths instead of as data

[hay]

When entering a PosixPath (e.g. from using pathlib) as input to a new Knead instance because it is not a string dataknead will think this is Python data (a native type), and will give back a stringified path as data instead of the file contents. This should be changed so that instances of PosixPath are recognised and treated as file paths.