pat-validation: Allow for HTML5 style required attributes without a value

[thet]

The HTML5 specs allow for an required attribute without a value. We're not supporting this in pat-validation until now. This PR fixes this.

Now we can use:

<input type="text" name="req1" required="required" />

as well as

<input type="text" name="req1" required />

See:
https://html.spec.whatwg.org/multipage/input.html#attr-input-required
https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/required

[pysailor]

Looks sane to me. The one failing test does not seem related (pat-tooltip: When a tooltip is clicked, if the 'source' parameter is 'content'…).
But I don't know how to trigger re-running the tests via a GH comment.

[pysailor]

But I don't know how to trigger re-running the tests via a GH comment.

Of course, directly in travis, this is possible. I just restarted the build