Fix filter config key

`App::className()` returns a namespaced class name, which then doesn't match any filter configuration key.
Fix wrong key in test mock

The actual key that is parsed (i.e. LessCss) does not include the namespace.
Fix missing directory separator
Handle cases when clearing unexistent cache
Fix Folder namespace
Merge pull request #227 from jadb/3.0-various-fixes

3.0 various fixes