patch for a bug inside of solr since solr 5.0 when using a boost

function with a numeric date field:
"unexpected docvalues type NUMERIC for field 'last_modified' (expected
one of [SORTED, SORTED_SET]). Use UninvertingReader or index with
docvalues."
This is a well-known bug inside solr which prevents that now the 'sort
by date' in the YaCy search interface can be used. Without this patch no
results at all is displayed (since the exception prevents that). Now
there is at least a result but it is not ordered properly.

source/net/yacy/cora/federate/solr/Ranking.java

@@ -177,6 +177,7 @@ public class Ranking {
      * @return
      */
     public String getBoostFunction() {
+        if (this.boostFunction.contains("last_modified")) return ""; // since solr 5.0 this does not work any more
         return this.boostFunction;
     }