Xhtml5Sink.date_() emits uncommon meta name

[kwin]

Affected version

2.0.0

Bug description

The date is emitted as <meta name="date" content="2013-01-02" > which is not a known name according to https://html.spec.whatwg.org/multipage/semantics.html#the-meta-element. Although https://maven.apache.org/doxia/doxia/doxia-sink-api/apidocs/org/apache/maven/doxia/sink/Sink.html#date-- is quite fuzzy about the semantics I think instead emitting one of the meta extensions from https://wiki.whatwg.org/wiki/MetaExtensions makes more sense.