Fatal error when casting variadic ReflectionParameter to string

``` php
function foo(...$a) {}

$f = new ReflectionFunction('foo');
$p = $f->getParameters()[0];
echo $p;
```

Gives:

> Fatal error: Uncaught exception 'ReflectionException' with message 'Parameter is not optional'

See https://3v4l.org/YsCPJ