Gets a JSON version of a User-Agent and uses a cached version for a slight speed advantage.
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);
    })
  }