XMLWriter throws ClassCastException on writing maps other than <String,?>

http://lucene.apache.org/solr/api/org/apache/solr/response/SolrQueryResponse.html#returnable_data says that a Map "containing any of the items in this list" may be contained in a SolrQueryResponse and will be handled by QueryResponseWriters.

This is not true for (at least) Keys in Maps.
XMLWriter tries to cast keys to Strings.