Fixed an error in "copy_r" method where I couldn't pass as a parameter a...

Fixed an error in "copy_r" method where I couldn't pass as a parameter a single file to be copied.

This code was throwing an error:

<pre>
fs.cp_r "path/to/single/file.yml", "path/to/single/copy.yml"
</pre>

The error message:

<pre>
fs.js:500
  return binding.readdir(pathModule._makeLong(path));
                 ^
Error: ENOTDIR, not a directory 'path/to/single/file.yml'
</pre>