If a user tries to create a new folder with the same title as another of his folders, an alert is shown.
Acceptance test to ensure that a user can create a new folder with the same title as a folder that belongs to another user.
FoldersController validates that the current user is subscribed to the feed passed in the params, before creating a new folder and adding the feed to it. If the user is not subscribed to the folder, returns a 404.
Fix an error in the code that tests if the user is subscribed to the passed feed when creating a new folder (the boolean condition was inverted). Add format: :json argument that ensures that the new folder form is POSTed to the JSON path.