Don't lowercase filenames on Linux before opening the file

This fixes https://jira.sil.org/browse/BL-828 "[Linux] Vaccination Shell
has lost the License info for Front Cover Image".  This inspired me to
go looking for similar problems elsewhere in the Bloom code.  The actual
fix for the code is in ImageUpdater.cs.
This seems a bit backward: PathEndingInSlash is already lowercase, so I'd think that if we want to do this we'd want to use url.ToLower().