make extending Stream with mixins easier

lib/sinatra/base.rb

@@ -243,9 +243,11 @@ module Sinatra
 
       def initialize(scheduler = self.class, close = true, &back)
         @back, @scheduler, @callback, @close = back.to_proc, scheduler, nil, close
+        super()
       end
 
       def close
+        return super if defined? super
         @scheduler.schedule { @callback.call if @callback }
       end
 
@@ -267,10 +269,13 @@ module Sinatra
       end
 
       def callback(&block)
+        return super if defined? super
         @callback = block
       end
 
-      alias errback callback
+      def errback
+        super if defined? super
+      end
     end
 
     # Allows to start sending data to the client even though later parts of