code: function(cb) {
      if (Stripe.USER_AGENT_SERIALIZED) {
        return cb(Stripe.USER_AGENT_SERIALIZED);
      }
      this.getClientUserAgentSeeded(Stripe.USER_AGENT, function(cua) {
        Stripe.USER_AGENT_SERIALIZED = cua;
        cb(Stripe.USER_AGENT_SERIALIZED);
      })
    }
docs: Gets a JSON version of a User-Agent and uses a cached version for a slight speed advantage.