Templates allowing asynchronous rendering are never displayed because of a malformed placeholder

A template that allows asynchronous rendering (with the ##!asyncAllowed=true header) is never displayed when it is rendered asynchronously: the HTML placeholder that should be replaced by the template content once it is ready is malformed, so the content is never loaded.

Steps to reproduce

1. Create the file templates/asynctest.vm in the XWiki web application (e.g. webapps/xwiki/templates/asynctest.vm in the Jetty/HSQLDB distribution) with the following content:

   ##!asyncAllowed=true
   <p id="asyncTestContent">Asynchronous template content</p>
   

2. Open /xwiki/bin/view/Main/?xpage=asynctest in the browser.

Expected result

The text "Asynchronous template content" is displayed, either directly or after it has been loaded asynchronously.

Actual result

The text is never displayed. The response contains a broken placeholder where the attributes are not separated, not quoted and the tag is self-closing:

<div class=xwiki-asyncdata-xwiki-async-id=template/environment%253A%252Ftemplates%252Fasynctest.vm/html%252F5.0/false/author/xwiki%253AXWiki.superadmin/secureDocument/null/RYULT8tS1data-xwiki-async-client-id=RYULT8tS1/>

Since the element doesn't have the xwiki-async class, the content is never fetched. Opening /xwiki/bin/view/Main/?xpage=asynctest&async=false displays the content as expected.