fix #14: unbalanced quotes in attribute value

This fixes the case for mis-balanced quotes in attribute values.

    <div class="hello id="world"> => open tag

This improves the tokenizer (all tests are passing) while beeing a little more html5 compatible as misplaced quotes are supposed to be discarded (check https://html.spec.whatwg.org/multipage/syntax.html#attribute-name-state