(PUP-1151) Respond with 404 when path not known

The v2.0 api implementation had a type that was not caught by any tests.
This fixes that issue (a typo) and adds a test that would have shown the
problem.
Merge pull request #2297 from zaphod42/issue/master/pup-1151-error-when-generating-404-response

(PUP-1151) Respond with 404 when path not known