Update item_property to recognize integers

lib/jekyll/filters.rb

@@ -368,15 +368,18 @@ def item_property(item, property)
       end
     end
 
-    # rubocop:disable Performance/RegexpMatch
+    FLOAT_LIKE   = %r!\A\s*-?(?:\d+\.?\d*|\.\d+)\s*\Z!.freeze
+    INTEGER_LIKE = %r!\A\s*-?\d+\s*\Z!.freeze
+    private_constant :FLOAT_LIKE, :INTEGER_LIKE
+
     # return numeric values as numbers for proper sorting
     def parse_sort_input(property)
-      number_like = %r!\A\s*-?(?:\d+\.?\d*|\.\d+)\s*\Z!
-      return property.to_f if property.to_s =~ number_like
+      stringified = property.to_s
+      return property.to_i if INTEGER_LIKE.match?(stringified)
+      return property.to_f if FLOAT_LIKE.match?(stringified)
 
       property
     end
-    # rubocop:enable Performance/RegexpMatch
 
     def as_liquid(item)
       case item

test/test_filters.rb

@@ -831,6 +831,16 @@ def to_liquid
           )
         end
       end
+
+      should "should pass integers as is" do
+        grouping = @filter.group_by([
+          { "name" => "Allison", "year" => 2016 },
+          { "name" => "Amy", "year" => 2016 },
+          { "name" => "George", "year" => 2019 },
+        ], "year")
+        assert_equal "2016", grouping[0]["name"]
+        assert_equal "2019", grouping[1]["name"]
+      end
     end
 
     context "where filter" do