take 2, works when it's not the immediate super call in the middle ch…

…ain, but requires an explicit option

example/base_app.ru

@@ -1,5 +1,6 @@
 require 'rack/request'
 require 'rack/response'
+require 'rack/showexceptions'
 require 'rack/session/cookie'
 require File.dirname(__FILE__) + '/../lib/rack-flash'
 
@@ -15,7 +16,7 @@ class Base
     res.finish
   end
 end
-
 use Rack::Session::Cookie
-use Rack::Flash
+use Rack::Flash, :flash_app_class => Base
+use Rack::ShowExceptions
 run Base.new

lib/rack/flash.rb

@@ -101,14 +101,11 @@ class << self; self end.class_eval do
     # - Rack Middleware implementation
 
     def initialize(app, opts={})
-      if defined?(Sinatra::Base)
-        Sinatra::Base.class_eval do
+      if app_class = opts[:flash_app_class] || defined?(Sinatra::Base) && Sinatra::Base
+        app_class.class_eval do
           def flash; env['rack-flash'] end
         end
-      else
-        def app.flash; env['rack-flash'];end
       end
-
       @app, @opts = app, opts
     end