Writing test for CacheHelper

Hi,

I'm working to patch actionpack/lib/action_view/helpers/
cache_helper.rb so that it doesn't throw when ouput_buffer isn't
ActionView::OutputBuffer but is still html_safe. In 3-0-stable the
function contains the following if:

          if output_buffer.is_a?(ActionView::OutputBuffer)
            safe_output_buffer = output_buffer.to_str
            fragment = safe_output_buffer.slice!(pos..-1)
            self.output_buffer =
ActionView::OutputBuffer.new(safe_output_buffer)
          else
            fragment = output_buffer.slice!(pos..-1)
          end

I have prepared a patch for this [1] which based on [2]. In addition,
there is currently a test in caching_test.rb called
test_fragment_caching_in_partials. This test would suffice as a
template for me but I need to find a way to change the output_buffer
in the helper. Any ideas how I could achieve this?

-Lauri

[1]: https://github.com/rails/rails/pull/2080
[2]: https://github.com/rails/rails/issues/1537