Java API: ScriptSortBuilder does not pass the lang used, closes elastic#1569.

Author: kimchy

modules/elasticsearch/src/main/java/org/elasticsearch/search/sort/ScriptSortBuilder.java

@@ -97,6 +97,9 @@ public ScriptSortBuilder lang(String lang) {
         if (order == SortOrder.DESC) {
             builder.field("reverse", true);
         }
+        if (lang != null) {
+            builder.field("lang", this.lang);
+        }
         if (this.params != null) {
             builder.field("params", this.params);
         }