Cannot set HTML class attribute on block element

It looks like support for html class attributes should work on block elements [here](https://github.com/basecamp/trix/blob/3614f3317b4239b7ed0925d9307f4acbba45a442/src/trix/core/helpers/dom.js#L199-L203) by passing a `className` option.

```
if (options.className) {
options.className.split(" ").forEach((className) => {
element.classList.add(className)
})
}
```

However I cannot get the editor to add a class to the element:
```
Trix.config.blockAttributes.h2 = {
tagName: 'h2',
className: 'test1'
}

addEventListener("trix-initialize", function(event) {
var buttonHTML = 'H2'
event.target.toolbarElement
.querySelector(".button_group.text_tools")
.insertAdjacentHTML("beforeend", buttonHTML)
})
```

##### Steps to Reproduce

1. Go to JS fiddle: https://jsfiddle.net/mzsjpn8e/8/
2. enter text into the editor
3. highlight the text
4. click the `h2` button
5. inspect the source code to see that the h2 element was create but that there is no class attribute
```

test

```
I am using actionText in rails. I see that the class attribute should already be allowed:
```
3.1.3 :001 > ActionText::ContentHelper.allowed_attributes
=>
#
```