format :json is not scoped to one API, it affects other APIs

We have two endpoints, one accepts application/json content-type, the other accepts text/plain. They live together. However, when format :json is included in one of our API's the other one is effected as well.

Consider the example below:

class API < Grape::API
  format :json
  get '/v1/users' do
    {'api': 'api'}
  end
end

class BAPI < Grape::API
  post '/v1/users' do
    {'bapi': 'bapi'}
  end
end

application = Rack::Builder.new do
  map '/' do
    run Rack::Cascade.new [API, BAPI]
  end
end

In this case, POST /v1/users returns {"error":"The requested content-type 'text/plain' is not supported."}. When format :json is removed, it returns {"bapi": "bapi"} as expected.

Reproduce the Rack::Builder example with API and BAPI to confirm that format :json affects the other endpoint's content-type handling. Trace how format configuration is shared between the two APIs; the work is done when each API keeps its own content-type behavior and the example no longer rejects text/plain for BAPI.