Allow tag classes to be reloaded when using Liquid.cache_classes is false

Because Liquid keeps a reference to tag classes, Rails class reloading may
cause problems with custom tags. This commit introduces a setting that
allows these classes to be resolved when required.

test/unit/template_unit_test.rb

@@ -16,4 +16,54 @@ class TemplateUnitTest < Test::Unit::TestCase
     assert_instance_of I18n, t.root.options[:locale]
     assert_equal fixture("en_locale.yml"), t.root.options[:locale].path
   end
+
+  def test_with_cache_classes_tags_returns_the_same_class
+    original_cache_setting = Liquid.cache_classes
+    Liquid.cache_classes = true
+
+    original_klass = Class.new
+    Object.send(:const_set, :CustomTag, original_klass)
+    Template.register_tag('custom', CustomTag)
+
+    Object.send(:remove_const, :CustomTag)
+
+    new_klass = Class.new
+    Object.send(:const_set, :CustomTag, new_klass)
+
+    assert Template.tags['custom'].equal?(original_klass)
+  ensure
+    Object.send(:remove_const, :CustomTag)
+    Template.tags.delete('custom')
+    Liquid.cache_classes = original_cache_setting
+  end
+
+  def test_without_cache_classes_tags_reloads_the_class
+    original_cache_setting = Liquid.cache_classes
+    Liquid.cache_classes = false
+
+    original_klass = Class.new
+    Object.send(:const_set, :CustomTag, original_klass)
+    Template.register_tag('custom', CustomTag)
+
+    Object.send(:remove_const, :CustomTag)
+
+    new_klass = Class.new
+    Object.send(:const_set, :CustomTag, new_klass)
+
+    assert Template.tags['custom'].equal?(new_klass)
+  ensure
+    Object.send(:remove_const, :CustomTag)
+    Template.tags.delete('custom')
+    Liquid.cache_classes = original_cache_setting
+  end
+
+  class FakeTag; end
+
+  def test_tags_delete
+    Template.register_tag('fake', FakeTag)
+    assert_equal FakeTag, Template.tags['fake']
+
+    Template.tags.delete('fake')
+    assert_nil Template.tags['fake']
+  end
 end