(PUP-716) Assert path before making stat expectation

File system changes ensure that Strings get converted to Pathnames; so
the Puppet::Util::Windows::File.stat call needs to expect a Pathname now.
Merge pull request #2244 from jpartlow/issue/master/pup-716-win2003-lstat-spec-fix

(PUP-716) Assert path before making stat expectation