dropbox: Fix crypt+obfuscate on dropbox - fixes #2191

Before this change we lowercased the dropbox root directory.  This was
likely a leftover from when we used to build a dictionary to translate
the cases of dropbox files.  Now with the v2 API we can rely on
dropbox to do that for us, so we no longer need to lowercase the root.

This fixes issues using crypt with name obfuscation on dropbox.

backend/dropbox/dropbox.go

@@ -240,11 +240,9 @@ func NewFs(name, root string) (fs.Fs, error) {
 // Sets root in f
 func (f *Fs) setRoot(root string) {
 	f.root = strings.Trim(root, "/")
-	lowerCaseRoot := strings.ToLower(f.root)
-
-	f.slashRoot = "/" + lowerCaseRoot
+	f.slashRoot = "/" + f.root
 	f.slashRootSlash = f.slashRoot
-	if lowerCaseRoot != "" {
+	if f.root != "" {
 		f.slashRootSlash += "/"
 	}
 }