Make NodeCache compliant con the Map interface on future releases

[serpi90]

https://262.ecma-international.org/7.0/#sec-properties-of-the-map-prototype-object

This would allow node-cache to be a drop-in replacement to things that expect a map, for example lodash's memoize which explains:

Note: The cache is exposed as the cache property on the memoized function. Its creation may be customized by replacing the _.memoize.Cache constructor with one whose instances implement the Map method interface of clear, delete, get, has, and set.

I currently have a workaround by doing:

const _ = require('lodash');
const NodeCache = require('node-cache');

// Adapter for NodeCache to have the Map interface expected by lodash.memoize
class MemoizeCache {
  constructor(args) { this.cache = new NodeCache(args); }
  clear() { this.cache.flushAll(); }
  delete(key) { return this.cache.del(key); }
  get(key) { return this.cache.get(key); }
  has(key) { return this.cache.has(key); }
  set(key, value) { this.cache.set(key, value); return this; }
}

module.exports = (fn, ttl) => {
  const memoized = _.memoize(fn);
  memoized.cache = new MemoizeCache({ stdTTL: ttl });
  return memoized;
};

But it would be nice to conform to that interface and not requiring that change.