Avoid collapsing `code` inline tag

I think there's a bug in current version:

```javascript
    var input = 'a <code>hello world</code> example.';
    var output = minify(input, { collapseWhitespace: true });

    output; // 'a<code>hello world</code>example.'
```

`code` is an inline element, collapsing should be avoided.