fixing test for XMLUnit 1.5 breaking changes

As per [XMLUnit 1.5 Release notes](http://xmlunit.sourceforge.net/userguide/html/apas05.html), it introduces 2 new differences if one node in the comparison has children while the other one has not.  For control HTML:
```html
<html><body><header>Crawljax</header><p>There are differences</p></body></html>
```
...and test HTML:
```html
<html><head><title>Crawljax</title></head><body><p>There are differences.</body></html>
```
... following are the 2 newly added differences apart from old 5:
* `Expected number of child nodes '0' but was '1' - comparing <HEAD...> at /HTML[1]/HEAD[1] to <HEAD...> at /HTML[1]/HEAD[1]` - **CHILD_NODELIST_LENGTH difference**
* `Expected presence of child node 'null' but was 'TITLE' - comparing  at null to <TITLE...> at /HTML[1]/HEAD[1]/TITLE[1]` - **CHILD_NODE_NOT_FOUND difference**