Improve copy/move target path handling

[andersevenrud]

Right now the API expects explicit target filenames (i.e. foo:/file.txt -> foo:/bar/file.txt) in order for these operations to work.

It would be nice to have the resolver apply the filename on the destination if it was not given (i.e. foo:/file.txt -> foo:/).

See also: os-js/osjs-client#80