Ensure sum filter calls to_liquid

lib/liquid/standardfilters.rb

@@ -878,19 +878,22 @@ module Liquid
     # @liquid_return [number]
     def sum(input, property = nil)
       ary = InputIterator.new(input, context)
+      return 0 if ary.empty?
 
-      if ary.empty?
-        0
-      elsif property.nil?
-        ary.sum do |item|
-          Utils.to_number(item)
-        end
-      else
-        ary.sum do |item|
-          item.respond_to?(:[]) ? Utils.to_number(item[property]) : 0
-        rescue TypeError
-          raise_property_error(property)
+      values_for_sum = ary.map do |item|
+        if property.nil?
+          item
+        elsif item.respond_to?(:[])
+          item[property]
+        else
+          0
         end
+      rescue TypeError
+        raise_property_error(property)
+      end
+
+      InputIterator.new(values_for_sum, context).sum do |item|
+        Utils.to_number(item)
       end
     end
 

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)