Incorrect results for some elements missing attributes

[aaronpk]

There are a few cases where the parser is returning incorrect results because of the way it tests for the presence of attributes.

http://pin13.net/mf2/?id=20170524175046008

<article class="h-entry">
  <a class="u-url">link</a>
</article>

currently parses as

        {
            "type": [
                "h-entry"
            ],
            "properties": {
                "url": [
                    "http://example.com/"
                ],
                "name": [
                    "link"
                ]
            }
        }

The correct result should have the url property set to link.

This is caused by this incorrect conditional check: $u->getAttribute('href') !== null

The getAttribute method will never return null according to the PHP docs: http://php.net/manual/en/domelement.getattribute.php

The code should instead be testing for the presence of the attribute with hasAttribute