Fix dict lookup

Charmhub sometimes sends Revision (with an "R"), sometimes sends
revision (with an "r") to keep things spicy

Author: cderici

juju/application.py

@@ -750,8 +750,9 @@ async def refresh(
 
             # no, if we already have it (and upstream doesn't have a newer res available)
             if res_name in existing_resources:
-                available_rev = resource['revision']
-                existing_rev = existing_resources[res_name].unknown_fields.get('revision', -1)
+                available_rev = resource.get('Revision', resource.get('revision', -1))
+                u_fields = existing_resources[res_name].unknown_fields
+                existing_rev = u_fields.get('Revision', resource.get('revision', -1))
                 if existing_rev >= available_rev:
                     continue