fixed DOMNode comparator for canonical comparison

https://github.com/sebastianbergmann/phpunit/commit/7dadedf6c770feb18c0e17d895e698d57a6fb9bb introduced with Release 4.0.18 a bug when comparing two DOMNodes.

Previously, the comparison was always canonical (thanks to C14N), but now it no longer is. I would call that a BC break. This PR is basically a revert of that commit to make the DOMNode comparator canonical again. The test should make the problem pretty clear:
```php
    public function testAssertEqualsNormalizesAttributes()
    {
        $comparator = new PHPUnit_Framework_Comparator_DOMNode();

        $document1 = new DOMDocument();
        $document1->loadXML("<a x='' a=''/>");

        $document2 = new DOMDocument();
        $document2->loadXML("<a a='' x=''/>");

        $comparator->assertEquals($document1, $document2);    // this fails since 4.0.18
    }
```

Realworld example: https://github.com/brianium/paratest/pull/90