function isKeyable(value) {
      var type = typeof value;
      return (type == 'string' || type == 'number' || type == 'symbol' || type == 'boolean')
        ? (value !== '__proto__')
        : (value === null);
    }
Checks if `func` is capable of being masked.

@private
@param {*} value The value to check.
@returns {boolean} Returns `true` if `func` is maskable, else `false`.