Wrong Location header if the POST url has no trailing slash

The new documents uri is resolved using ```url.resolve(from, to)```, which replaces what is after the last slash by the to part (like a browser resolves ```href``` urls)
  - POST /api/users  => Location: /api/123456
  - POST /api/users/ => Location: /api/users/123456
Merge pull request #74 from feychenie/patch-2

Wrong Location header if the POST url has no trailing slash