Do not treat draggable as a boolean attribute.

While the values of draggable are "true" and "false" (and "auto"), it does not behave like other Boolean attributes and is actually classified as an [enumerated attribute](http://drafts.htmlwg.org/html/master/editing.html#the-draggable-attribute).

For example, currently this code
```html
<div draggable="true"></div>
```
...would be minified as
```html
<div draggable></div>
```
...which is actually equivalent to
```html
<div draggable="false"></div>
```
This is obviously incorrect.