Fix line breaks serialization in list items

[zhouzi]

Right now, line breaks in list items are not properly serialized. The following document:

<ul>
	<li>Hello\nWorld</li>
</ul>

Is serialized to:

* Hello World

Which is not rendered as a line-break nor will it be parsed as such so the line break is lost. The way line-breaks are treated in list items is a bit complex so we'd need to add more tests and maybe tweak the one that's currenly added. The specification can be found here:

• List Items Spec