fixed ScalatraServlet.requestPath for encoded unicode URI

There was problem with unicode servlet mappings like this

```scala
context.mount(new TestServlet, "тест")
```

The issue is that requestPath tried first remove context and servlet paths from the uri and then decode the result, it works for latin servlet paths, but failed for unicode. You need first decode the uri and then remove context and servlet paths from it to result the requestPath.

Here is the fix and test for it.