Fix bug in getmetadata command: new_command/4 was allowing Folder to be a list, when the internal API required it to be binary

Summary:
the arity-4 version also needs to check for Folder being a list

this is now dead code, as new_command/4 handles the check for list now

Reviewers: #eimap_developers

Differential Revision: https://git.kolab.org/D170