epub3-to-dtbook: fixed error with multiple classes on the same p (FORG0006)

This would throw a FORG0006 error, but is now fixed:

```html
<hr class="emptyline"/>
<p class="indented">TEXT</p>
```

The result in DTBook contains multiple classes and should be:

```xml
<p class="indented precedingemptyline">TEXT</p>
```