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

This would throw a FORG0006 error, but is now fixed:

```html
<hr class="emptyline"/>
<p class="indented">TEXT</p>
```

The result in DTBook is contains multiple classes and should be:

```xml
<p class="indented precedingemptyline">TEXT</p>
```
Merge pull request #138 from nlbdev/forg0006-multiple-class-error-fix

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