dropbox: make setting mod time on existing files work properly

This is a fix left over from the v2 conversion.  Dropbox ignores the
client modification on an incoming file if it was identical to the
existing file.  This change deletes the existing file first before
re-uploading the new one.

fstest/fstests/fstests.go

@@ -672,7 +672,7 @@ func TestObjectSetModTime(t *testing.T) {
 	newModTime := fstest.Time("2011-12-13T14:15:16.999999999Z")
 	obj := findObject(t, file1.Path)
 	err := obj.SetModTime(newModTime)
-	if err == fs.ErrorCantSetModTime {
+	if err == fs.ErrorCantSetModTime || err == fs.ErrorCantSetModTimeWithoutDelete {
 		t.Log(err)
 		return
 	}