Enable incrementally building on top of a previous API version

[dblock]

API v1

module Acme
  class V1 < Grape::API
    format :json
    version [ 'v2', 'v1' ], using: :header, vendor: 'acme', format: :json, cascade: true

    desc "Returns the current API version, v1."
    get do
      { version: 'v1' }
    end

    desc "Returns pong."
    get "ping" do
      { ping: "pong" }
    end
  end
end

API v2

module Acme
  class V2 < Grape::API
    format :json
    version 'v2', using: :header, vendor: 'acme', format: :json, cascade: true

    desc "Returns the current API version, v2."
    get do
      { version: 'v2' }
    end
  end
end

We should be able to mount these in v2, then v1 order, like so:

Rack::Cascade.new([ Acme::V2, Acme::V1 ])

Then V2 will respond to API requests that it implements, and V1 will respond to anything remaining, since it also supports v2.

[dblock]

Fixed in 2be499c and wrote an example of how to fallback to the implementation of a previous version: https://github.com/dblock/grape-on-rack-v1-inside-v2.