call (Camping::Server::XSendfile)

    # File lib/camping/server.rb, line 239
          def call(env)
            status, headers, body = @app.call(env)

            if key = headers.keys.grep(/X-Sendfile/i).first
              filename = headers[key]
              content = File.read(filename)
              headers['Content-Length'] = size(content).to_s
              body = [content]
            end

            return status, headers, body
          end