serialToString fix for <tag/> non bachelor tags

When serializing back to text:

<div class=“”></div> was incorrectly getting truncated to <div
class=“’/>

I re-wrote the default to <tag attr="someatt"></tag> for all tags.
Then made the exceptions for "meta|link|img|br|hr|input", which will delete <hr>any text inside this space</hr>
Since those tags should only allow attributes.

I also wrote a unit test.

And also, thanks for this useless library! Cheers.