Thanks to the hard work of Skye Shaw, amongst others, this release of
rack-contrib supports Rack 2.x. Unfortunately, it only supports Rack
2.x; if your application is using Rack 1.x, you should continue to use
rack-contrib 1.x.

The non-backwards-compatible, user-visible changes are:

• Drop support for Ruby versions less than 2.2. Rack 2 does not support
  these older releases, so there's no benefit in our doing so.

• Rack::NestedParams: switch to using Rack::Utils.parse_nested_query to
  parse request bodies, which handles repeated element keys differently.
  See #92.

• Rack::Sendfile: removed completely. Rack core provides a middleware of
  the same name that is much better, and more actively maintained, and you
  should use that instead.

• Rack::AcceptFormat: removed completely, because it is terribad.