Treat attribtues set with boolean values as properties

[weotch]

When applying a checked state to a checkbox, it is convenient to set the checked attribute with a boolean like the jQuery prop() API.

$bool = false; // Perhaps the property of a model
$el = Input::checkbox('foo', 'bar', ['checked' => $bool]);

#14 / #15 broke this by making NULL be the only wait to pass an attribute value that tells html-object not to add the attribute. This PR makes it so setting an attribute value to FALSE will NOT create the attribute. And setting it to TRUE will add the attribute as a property (no value).

[RTC1]

Looks like this solves #20

[biwerr]

Please don't treat the value attribute as property. This has some issues if you have checkbox values with true/false checkbox will not be send via form post if there is no value specified.

HtmlObject\Traits\Helpders@parseAttributes

// If set to boolean true, make the attribtue a property
            if ($value === true && $key!=='value') {
                $html[] = $key;
                continue;
            }