call (Merb::Rack::ContentLength)

    # File lib/merb-core/rack/middleware/content_length.rb, line 7
          def call(env)
            status, headers, body = @app.call(env)

            # to_s is because Rack spec expects header
            # values to be iterable and yield strings
            header = 'Content-Length'.freeze
            headers[header] = body.size.to_s unless headers.has_key?(header)

            [status, headers, body]
          end