Fix custom tag render result stringification

lib/liquid/tag.rb

@@ -58,7 +58,10 @@ def render(_context)
     # method will be removed.
     def render_to_output_buffer(context, output)
       render_result = render(context)
-      output << render_result if render_result
+      if render_result
+        render_result = render_result.join if render_result.is_a?(Array)
+        output << render_result.to_s
+      end
       output
     end
 

test/integration/tag_test.rb

@@ -42,4 +42,30 @@ def render(*)
       assert_equal(buf.object_id, output.object_id)
     end
   end
+
+  def test_custom_tags_stringify_render_result_for_backwards_compatibility
+    integer_tag = Class.new(Tag) do
+      def render(*)
+        49
+      end
+    end
+
+    with_custom_tag('blabla', integer_tag) do
+      template = Liquid::Template.parse("{% blabla %}")
+
+      assert_equal('49', template.render)
+    end
+
+    array_tag = Class.new(Tag) do
+      def render(*)
+        [4, 9]
+      end
+    end
+
+    with_custom_tag('blabla', array_tag) do
+      template = Liquid::Template.parse("{% blabla %}")
+
+      assert_equal('49', template.render)
+    end
+  end
 end