new_insert_before.rb

def insert_before(klass, options={})
  instance     = new(nil, options)
  call_without = "call_without_#{instance.object_id}"
  call_with    = "call_with_#{instance.object_id}"

  klass.class_eval do
    define_method call_with do |env|
      instance.app = proc { |env| send call_without, env }
      instance.call(env)
    end
    alias_method_chain :call, instance.object_id.to_s
  end
end

old_insert_before.rb

def insert_before(klass, options={})
  instance = new(nil, options)

  klass.class_eval do
    unbound_call = instance_method(:call)
    define_method(:call) do |env|
      instance.app = unbound_call.bind(self)
      instance.call(env)
    end
  end
end