constraints will be overrided when multiple router created

[climba03003]

The current code override the HandlerStorage.prototype.constrainer to get the constrainter in router.
It will be override when two store created first before any route trying to lookup.

const findMyWay = require("find-my-way");

const constraint = {
  name: "secret",
  storage: function () {
    let secrets = {};
    return {
      get: (secret) => {
        return secrets[secret] || null;
      },
      set: (secret, store) => {
        secrets[secret] = store;
      },
      del: (secret) => {
        delete secrets[secret];
      },
      empty: () => {
        secrets = {};
      },
    };
  },
  deriveConstraint: (req, ctx) => {
    return req.headers["x-secret"];
  },
  validate() {
    return true;
  },
};

let router1 = findMyWay({ constraints: { secret: constraint } });

// it override the constraints of router1
const router2 = findMyWay({ constraints: undefined });

// it can registered constraints
router1.on("GET", "/", { constraints: { secret: "alpha" } }, () => {});
// it cannot find the constraints secret
router1.find("GET", "/", { constraints: { secret: "alpha" } });