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.

:create_multipart_upload and :object_url are probably the only options
that will be used, so it makes sense to show them in the examples.

This allows defining other /s3/* routes without potential clashes when
Uppy::S3Multipart::App isn't mounted last. One example of such route is
"/s3/params", which is called by the AWS S3 Uppy plugin.

We still keep backwards compatibility for users that currently have it
mounted on "/s3".

It's shorter, more intention revealing, and works across all browsers.

That should always work, regardless of whether the app is running
locally or remotely, so users can just copy-paste this.

We also switch to the raw XML format, as I think it's easy enough to
make the update through the AWS S3 Console after all.

Closes #1