Fixes directory escaping bug in FileServer

Closes #37
Improves escape path detection, returns 400

- Now requests like "/abc../abc./abc.md" pass test.
- Any requests that do have "/../" escapes are served a 400: Bad Request
- Abstracted detection to function so it can be unit tested
Changes escape detection to normalized prefix comp

@astrieanna makes a good case for wanting to preserve legitimate uses of '/../' that do not result in directory escape.  Results in 1 less problem.