Fix bug in list_files()

Fixed a bug in the list_files function that stopped the url ever being correctly passed to get_files_with_prefix. Passing a path as an argument to list_files now behaves as expected.

Old behaviour:
``` list_files('pages'); ```

would by default list the current directory, not the pages subdirectory.

New behaviour: 
``` list_files('pages'); ```

now correctly lists the pages subdirectory.