use Proc.new to reduce some conditionals

Proc.new will pick up the passed in block, but since it's a default
param, it won't get evaluated unless someone doesn't pass in an app.  It
will raise an exception if no block is provided.

actionpack/lib/action_dispatch/middleware/stack.rb

@@ -112,9 +112,7 @@ def use(*args, &block)
       middlewares.push(middleware)
     end
 
-    def build(app = nil, &block)
-      app ||= block
-      raise "MiddlewareStack#build requires an app" unless app
+    def build(app = Proc.new)
       middlewares.freeze.reverse.inject(app) { |a, e| e.build(a) }
     end