Ensure sum filter calls to_liquid

test/integration/standard_filter_test.rb

@@ -982,6 +982,18 @@ class StandardFiltersTest < Minitest::Test
     end
   end
 
+  def test_sum_without_property_calls_to_liquid
+    t = TestThing.new
+    Liquid::Template.parse('{{ foo | sum }}').render("foo" => [t])
+    assert(t.foo > 0)
+  end
+
+  def test_sum_with_property_calls_to_liquid_on_property_values
+    t = TestThing.new
+    Liquid::Template.parse('{{ foo | sum: "quantity" }}').render("foo" => [{ "quantity" => t }])
+    assert(t.foo > 0)
+  end
+
   private
 
   def with_timezone(tz)