Clarify changes made to Rack::RequestBody

While the Rack Spec doesn't need Servers to have a #to_io method for request bodies at the moment, Puma for instance does it currently: https://github.com/puma/puma/blob/v2.8.2/lib/puma/client.rb#L27.
Yahns, Unicorn and Rainbows don't even use standard ruby sockets but http://bogomips.org/kgio/ which are compatible but don't have a #to_io method for their request bodies.

To use this in a Webmachine::Resource
```ruby
class UploadResource < Webmachine::Resource
  def allowed_methods; %W[POST]; end

  def content_types_accepted
    [['multipart/form-data', :from_form]]
  end

  def process_post
    @request.body.to_io.each do |body|
      puts body
    end
    true
  end

  def from_form; end
end
```

This has the advantage that you don't cache the whole body in memory like the current rack adapter does it with
@request.body.each.

Please be aware: only the rack adapter has this #to_io method currently guaranteed for request bodies, others don't have it necessarily.