item.categories are not optional

My feed items do not have categories, and the docs list this as optional.

When running `feed.xml()`:
```
TypeError: undefined is not an object (evaluating 'item.categories.forEach')
at /node_modules/rss/lib/index.js:71:9
at forEach (:1:21)
at generateXML (/node_modules/rss/lib/index.js:63:5)
at /node_modules/rss/lib/index.js:185:17
```

When I change `node_modules/rss/lib/index.js:71` to:
```js
item.categories?.forEach(function(category) {
ifTruePush(category, item_values, { category: { _cdata: category } });
});
```

I am able to generate the xml feed