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 in ```from``` with the ```to``` part (like a browser resolves ```href``` urls)

To reproduce the bug :
  - POST /api/users  => Location: /api/123456
  - POST /api/users/ => Location: /api/users/123456

This fix adds a trailing slash to the url's ```basePath``` on a POST if it's missing