[FIX] Use canonical path for `File#exists()`

JUnit test `FNFileTest#move()` failed on OS X because OS X cannot resolve paths
like `./directory/file.txt/../file.txt`. `File#getCanonicalFile()` does, so
resolving using that function fixes the problem.
Merge pull request #674 from JensErat/filetest-fix

Fix FNFileTest-JUnit test on OS X