Support array of methods in route variants

[javierbrea]

It is desirable to support an array of methods in the route variants definitions. It makes sense for defining middlewares that should be executed in every method, as described in mocks-server/main#140.

Using the middleware example from the documentation, this is how it should look like:

module.exports = [
  {
    id: "add-headers",
    url: "*",
    method: ["GET", "POST", "PUT", "DELETE"],
    variants: [
      {
        id: "enabled",
        response: (req, res, next) => {
          res.set('Content-Type', 'application/json; charset=utf-8');
          next();
        }
      },
      {
        id: "disabled",
        response: (req, res, next) => next()
      }
    ]
  }  
];