Improved boolean attribute handling

[goetas]

This HTML code is serialized in a wrong way:

<select>
    <option value="">foo</option>
</select>

<div data-attr="">
</div>

Pre-fix result

<select>
    <option value>foo</option>
</select>

<div data-attr>
</div>

Post-fix result

<select>
     <option value="">foo</option>
</select>

<div data-attr="">
</div>

[coveralls]

Coverage increased (+0.75%) to 89.73% when pulling 93df365 on boolean-attribiutes into 6222ad4 on master.

[coveralls]

Coverage increased (+0.75%) to 89.73% when pulling 93df365 on boolean-attribiutes into 6222ad4 on master.

[coveralls]

Coverage increased (+0.75%) to 89.73% when pulling e36b070 on boolean-attribiutes into 6222ad4 on master.