feed.item errors when passed directly to Array.prototype.forEach

I have a static site generator which, shockingly, is using node-rss to output RSS feeds.

This code runs successfully:

```js
file.data.posts.map(function(post) {
return {
title: post.title,
url: urlPrefix + file.relative,
categories: post.categories,
// TODO: normalize URLs to absolute URLs
// See the `rss` docs for details
description: post.contents,
date: new Date(post.time.epoch * 1000)
};
}).forEach(function(item) {
feed.item(item);
});
```

As I'm sure you've guessed, `file.data.posts` is an array of posts. All that this does is map each post in to a node-rss-compatible item object and then call `.item()` on each object.

This code is theoretically functionally equivalent:

```js
file.data.posts.map(function(post) {
return {
title: post.title,
url: urlPrefix + file.relative,
categories: post.categories,
// TODO: normalize URLs to absolute URLs
// See the `rss` docs for details
description: post.contents,
date: new Date(post.time.epoch * 1000)
};
}).forEach(feed.item);
```

but it fails with `TypeError: Cannot read property 'items' of undefined`. Note that `feed.item` is being passed directly into `forEach` - presumably this is mangling `this` somehow, causing the error.

Here's the full stack trace:

```
/Users/alex/Development/github/strugee.github.com/node_modules/rss/lib/index.js:179
this.items.push(item);
^

TypeError: Cannot read property 'items' of undefined
at RSS.item (/Users/alex/Development/github/strugee.github.com/node_modules/rss/lib/index.js:179:13)
at Array.forEach (native)
at DestroyableTransform._transform (/Users/alex/Development/github/strugee.github.com/node_modules/stratic-indexes-to-rss/index.js:62:6)
at DestroyableTransform.Transform._read (/Users/alex/Development/github/strugee.github.com/node_modules/readable-stream/lib/_stream_transform.js:159:10)
at DestroyableTransform.Transform._write (/Users/alex/Development/github/strugee.github.com/node_modules/readable-stream/lib/_stream_transform.js:147:83)
at doWrite (/Users/alex/Development/github/strugee.github.com/node_modules/readable-stream/lib/_stream_writable.js:313:64)
at writeOrBuffer (/Users/alex/Development/github/strugee.github.com/node_modules/readable-stream/lib/_stream_writable.js:302:5)
at DestroyableTransform.Writable.write (/Users/alex/Development/github/strugee.github.com/node_modules/readable-stream/lib/_stream_writable.js:241:11)
at DestroyableTransform.ondata (/Users/alex/Development/github/strugee.github.com/node_modules/readable-stream/lib/_stream_readable.js:531:20)
at emitOne (events.js:96:13)
```

I'm on Node.js@4.4, node-rss@1.2.1.