Convert Strainer to white-list method protection

After moving the method existence check from Context into Strainer,
updated Strainer to only accept invokation methods that were added via
filter Modules, and done in a way that respond_to? is never called,
preventing unconstrained Symbol table growth.

lib/liquid/context.rb

@@ -71,11 +71,7 @@ module Liquid
     end
 
     def invoke(method, *args)
-      if strainer.respond_to?(method)
-        strainer.__send__(method, *args)
-      else
-        args.first
-      end
+      strainer.invoke(method, *args)
     end
 
     # Push new local scope on the stack. use <tt>Context#stack</tt> instead

lib/liquid/strainer.rb

@@ -9,16 +9,11 @@ module Liquid
   end
 
   # Strainer is the parent class for the filters system.
-  # New filters are mixed into the strainer class which is then instanciated for each liquid template render run.
+  # New filters are mixed into the strainer class which is then instantiated for each liquid template render run.
   #
-  # One of the strainer's responsibilities is to keep malicious method calls out
+  # The Strainer only allows method calls defined in filters given to it via Strainer.global_filter,
+  # Context#add_filters or Template.register_filter
   class Strainer < parent_object #:nodoc:
-    INTERNAL_METHOD = /^__/
-    @@required_methods = Set.new([:__id__, :__send__, :respond_to?, :kind_of?, :extend, :methods, :singleton_methods, :class, :object_id])
-
-    # Ruby 1.9.2 introduces Object#respond_to_missing?, which is invoked by Object#respond_to?
-    @@required_methods << :respond_to_missing? if Object.respond_to? :respond_to_missing?
-
     @@filters = {}
 
     def initialize(context)
@@ -36,19 +31,22 @@ module Liquid
       strainer
     end
 
-    def respond_to?(method, include_private = false)
-      method_name = method.to_s
-      return false if method_name =~ INTERNAL_METHOD
-      return false if @@required_methods.include?(method_name)
-      super
-    end
-
-    # remove all standard methods from the bucket so circumvent security
-    # problems
-    instance_methods.each do |m|
-      unless @@required_methods.include?(m.to_sym)
-        undef_method m
+    def invoke(method, *args)
+      if has_method?(method)
+        send(method, *args)
+      else
+        args.first
       end
     end
+
+    private
+
+    def has_method?(method)
+      methods_to_check = self.methods - self.class.public_instance_methods
+      methods_to_check.any? do |instance_method|
+        instance_method.to_s == method.to_s
+      end
+    end
+
   end
 end