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.
function mapCacheSet$3(key, value) {
	  var data = _getMapData(this, key),
	      size = data.size;

	  data.set(key, value);
	  this.size += data.size == size ? 0 : 1;
	  return this;
	}