Make Strainer also raise when registered method is overriden as protected

lib/liquid/strainer.rb

@@ -28,9 +28,9 @@ 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)
-        invokable_private_methods = filter.private_instance_methods.select { |m| invokable?(m) }
+        invokable_non_public_methods = (filter.private_instance_methods + filter.protected_instance_methods).select { |m| invokable?(m) }
-        if invokable_private_methods.any?
+        if invokable_non_public_methods.any?
-          raise MethodOverrideError, "Filter overrides registered public methods as private: #{invokable_private_methods.join(', ')}"
+          raise MethodOverrideError, "Filter overrides registered public methods as non public: #{invokable_non_public_methods.join(', ')}"
         else
           send(:include, filter)
           @filter_methods.merge(filter.public_instance_methods.map(&:to_s))

test/unit/strainer_unit_test.rb

@@ -102,7 +102,24 @@ class StrainerUnitTest < Minitest::Test
     error = assert_raises(Liquid::MethodOverrideError) do
       strainer.class.add_filter(PrivateMethodOverrideFilter)
     end
-    assert_equal 'Liquid error: Filter overrides registered public methods as private: public_filter', error.message
+    assert_equal 'Liquid error: Filter overrides registered public methods as non public: public_filter', error.message
+  end
+
+  module ProtectedMethodOverrideFilter
+    protected
+
+    def public_filter
+      "overriden as protected"
+    end
+  end
+
+  def test_add_filter_raises_when_module_overrides_registered_public_method_as_protected
+    strainer = Context.new.strainer
+
+    error = assert_raises(Liquid::MethodOverrideError) do
+      strainer.class.add_filter(ProtectedMethodOverrideFilter)
+    end
+    assert_equal 'Liquid error: Filter overrides registered public methods as non public: public_filter', error.message
   end
 
   module PublicMethodOverrideFilter