Implented to_str as fallback even if to_path should be available

In mri if a class defines to_str the String returned from this Method is
used as Pathname. In rbx you have to define to_path to get the same
behaviour.

This would use to_str as fallback if the Class provided as Path doesn't
implement to_path.
added fallback spec
don't pollute the namespace with fixture classes - use mock objects
Merge pull request #4 from Benny1992/2.0

Different behaviour between mri and rbx