[bfss] convert root BFSS directory to str in constructor (closes #2137793)
otherwise you can get crashed with AssertionError: Binary objects must use raw strings, not <type 'unicode'>
<div id="ajaxroot">
<div class="ajaxHtmlHead">
<script src="http://foo.js" type="text/javascript"> </script>
<link rel="stylesheet" type="text/css" media="all" href="qunit.css" />
</div>
<h1>Hello</h1>
</div>