Stop mutating body response

[matrinox]

If @app.call returns an object that is saved (for e.g., in a constant), the mutation results in a continuing cycle of wrapping the body in Rack::BodyProxy, eventually leading to SystemStackError. Here's an example to reproduce:

class HealthcheckApp
  SUCCESS_RESPONSE = [200, { 'Content-Type' => 'text/plain' }, ['success']].freeze

  def initialize(app)
    @app = app
  end

  def call(env)
    return SUCCESS_RESPONSE if env['PATH_INFO'] == '/heartbeat'
    @app.call(env)
  end
end

app = HealthcheckApp.new(-> (_x) { [200, {}, nil] })
logger = Rails::Rack::Logger.new(app)

logger.call('REQUEST_METHOD' => 'GET')

[rails-bot]

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[matrinox]

@rafaelfranca Which minor versions would get this fix? We are on 4.2.x