function mapCacheSet(key, value) {
  var data = getMapData(this, key),
      size = data.size;

  data.set(key, value);
  this.size += data.size == size ? 0 : 1;
  return this;
}
Sets the map `key` to `value`.

@private
@name set
@memberOf MapCache
@param {string} key The key of the value to set.
@param {*} value The value to set.
@returns {Object} Returns the map cache instance.