fields_for returns block result when outside ERB

Signed-off-by: wycats <wycats@gmail.com>

actionpack/lib/action_view/helpers/form_helper.rb

@@ -528,7 +528,7 @@ module ActionView
 
         builder = options[:builder] || ActionView::Base.default_form_builder
 
-        with_output_buffer do
+        capture do
           yield builder.new(object_name, object, self, options, block)
         end
       end

actionpack/test/template/form_helper_test.rb

@@ -1518,6 +1518,11 @@ class FormHelperTest < ActionView::TestCase
     assert_equal expected, output_buffer
   end
 
+  def test_fields_for_returns_block_result
+    output = fields_for(Post.new) { |f| "fields" }
+    assert_equal "fields", output
+  end
+
   protected
     def comments_path(post)
       "/posts/#{post.id}/comments"