Do not mutate options hash

actionpack/lib/action_view/helpers/record_tag_helper.rb

@@ -92,8 +92,8 @@ def content_tag_for(tag_name, single_or_multiple_records, prefix = nil, options
         # for each record.
         def content_tag_for_single_record(tag_name, record, prefix, options, &block)
           options, prefix = prefix, nil if prefix.is_a?(Hash)
-          options ||= {}
-          options.merge!({ :class => "#{dom_class(record, prefix)} #{options[:class]}".strip, :id => dom_id(record, prefix) })
+          options = options ? options.dup : {}
+          options.merge!(:class => "#{dom_class(record, prefix)} #{options[:class]}".strip, :id => dom_id(record, prefix))
           if block.arity == 0
             content_tag(tag_name, capture(&block), options)
           else

actionpack/test/template/record_tag_helper_test.rb

@@ -105,4 +105,10 @@ def test_content_tag_for_collection_is_html_safe
     result = content_tag_for(:li, [post_1, post_2]) { |post| post.body }
     assert result.html_safe?
   end
+
+  def test_content_tag_for_does_not_change_options_hash
+    options = { :class => "important" }
+    result  = content_tag_for(:li, @post, options) { }
+    assert_equal({ :class => "important" }, options)
+  end
 end