Ignore trailing slash when reading fake directories

See ticket #11330.
Fake: make sure that files are always listed in alphabetical order

Since only the last file in multi-file SPW datasets is used to
determine dimensions, it's important to always use the last file
alphabetically.  Otherwise, plates, wells, and/or fields could be
ignored.

Fixes ticket #11330.
Merge pull request #756 from melissalinkert/ticket-11330

Fix SPW fake file detection