correctly derive paths for UploadedFileUploader
correctly derive paths for UploadedFileUploader

`UploadedFileUploader`'s path generation was extremely fragile. for instance, it
would generate an incorrect path if the configuration option didn't end with
'/'.

to generate a vaild in a system independent way, use `Pathname#/` and then cast
back to a string.

samvera/hyrax,samvera/hyrax,samvera/hyrax,samvera/hyrax