ARIA related attributes are dropped by sanitizer

[tushuhei]

When I serialize a HTML document including elements with ARIA related attributes, serialize method drops them off as a result of sanitization.

import html5lib
doc = '<div id="foo">the description</div><span aria-describedby="foo">bar</span>'
print(html5lib.serialize(html5lib.parseFragment(doc),
                         sanitize=True, quote_attr_values='always'))
# <div id="foo">the description</div><span>bar</span>

ARIA related attributes are not dangerous for my understanding, so serialize method should keep them even if the parameter sanitize is True.