function mapHas(key) {
	      return key != '__proto__' && hasOwnProperty.call(this.__data__, key);
	    }
Checks if a cached value for `key` exists.

@private
@name has
@memberOf _.memoize.Cache
@param {string} key The key of the entry to check.
@returns {boolean} Returns `true` if an entry for `key` exists, else `false`.