Strainer#add_filter should raise when registered method is overriden as private

lib/liquid/errors.rb

@@ -59,4 +59,5 @@ module Liquid
   UndefinedVariable = Class.new(Error)
   UndefinedDropMethod = Class.new(Error)
   UndefinedFilter = Class.new(Error)
+  MethodOverrideError = Class.new(Error)
 end

lib/liquid/strainer.rb

@@ -28,8 +28,13 @@ module Liquid
     def self.add_filter(filter)
       raise ArgumentError, "Expected module but got: #{filter.class}" unless filter.is_a?(Module)
       unless self.class.include?(filter)
-        send(:include, filter)
-        @filter_methods.merge(filter.public_instance_methods.map(&:to_s))
+        invokable_private_methods = filter.private_instance_methods.select { |m| invokable?(m) }
+        if invokable_private_methods.any?
+          raise MethodOverrideError, "Filter overrides registered public methods as private: #{invokable_private_methods.join(', ')}"
+        else
+          send(:include, filter)
+          @filter_methods.merge(filter.public_instance_methods.map(&:to_s))
+        end
       end
     end