make Template override static registers only when the register key is not defined

lib/liquid/template.rb

@@ -212,7 +212,7 @@ module Liquid
       end
 
       Template.registers.each do |key, register|
-        context_register[key] = register
+        context_register[key] = register unless context_register.key?(key)
       end
 
       # Retrying a render resets resource usage

test/integration/template_test.rb

@@ -362,6 +362,50 @@ class TemplateTest < Minitest::Test
     assert_equal('12345', result)
   end
 
+  def test_render_uses_correct_disabled_tags_instance
+    Liquid::Template.file_system = StubFileSystem.new(
+      'foo' => 'bar',
+      'test_include' => '{% include "foo" %}'
+    )
+
+    disabled_tags = DisabledTags.new
+    context = Context.build(registers: { disabled_tags: disabled_tags })
+
+    source = "{% render 'test_include' %}"
+    parse_context = Liquid::ParseContext.new(line_numbers: true, error_mode: :strict)
+    document = Document.parse(Liquid::Tokenizer.new(source, true), parse_context)
+
+    assert_equal("include usage is not allowed in this context", document.render(context))
+  end
+
+  def test_render_sets_context_static_register_when_register_key_does_exist
+    disabled_tags_for_test = DisabledTags.new
+    Template.add_register(:disabled_tags, disabled_tags_for_test)
+
+    t = Template.parse("{% if true %} Test Template {% endif %}")
+
+    context = Context.new
+    refute(context.registers.key?(:disabled_tags))
+
+    t.render(context)
+
+    assert(context.registers.key?(:disabled_tags))
+    assert_equal(disabled_tags_for_test, context.registers[:disabled_tags])
+  end
+
+  def test_render_does_not_override_context_static_register_when_register_key_exists
+    context = Context.new
+    context.registers[:random_register] = nil
+    Template.add_register(:random_register, {})
+
+    t = Template.parse("{% if true %} Test Template {% endif %}")
+
+    t.render(context)
+
+    assert_nil(context.registers[:random_register])
+    assert(context.registers.key?(:random_register))
+  end
+
   unless taint_supported?
     def test_taint_mode
       assert_raises(NotImplementedError) do