req.header.keys { |key| header[key] = req.header[key][0] }

didn't work for me, to make it work I had to change the line to:

req.header.keys.each { |key| header[key] = req.header[key][0] }


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I also found that to fill the cookies in the WEBRick::HTTPResponse, in the
"Convert Net::HTTP::HTTPResponse to WEBrick::HTTPResponse" section, you need to do something like:

set_cookie(response, res)

  def set_cookie(src, dst)
    if str = src['set-cookie']
      cookies = []
      str.split(/,\s*/).each{|token|
        if /^[^=]+;/o =~ token
          cookies[-1] << ", " << token
        elsif /=/o =~ token
          cookies << token
        else
          cookies[-1] << ", " << token
        end
      }
      dst.cookies.replace(cookies)
    end
  end

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Just my 2 cents, thanks for sharing that snippet, it inspired me.
Keep up the good work!