code: def call!(env)
        @env = env.dup
        status, @headers, response = @app.call(@env)      
        if should_clean?
          @headers.delete('Content-Length')
          response = Rack::Response.new(
            tidy_markup(response.respond_to?(:body) ? response.body : response),
            status,
            @headers
          )
          response.finish
          response.to_a
        else
          [status, @headers, response]
        end
      end
docs: thread safe version using shallow copy of env