Unlike some other web frameworks like Roda, Rails adds a trailing slash
to PATH_INFO when calling mounted apps on root requests. This means that
for `POST /s3/multipart`, the Uppy::S3Multipart::App will receive

  SCRIPT_NAME: "/s3/multipart"
  PATH_INFO: "/"

instead of

  SCRIPT_NAME: "/s3/multipart"
  PATH_INFO: ""

So we update the app to handle that trailing slash.