Query DSL: Bool filter does not take should clauses properly into account, closes elastic#1511.

Author: kimchy

modules/elasticsearch/src/main/java/org/elasticsearch/common/lucene/search/XBooleanFilter.java

@@ -82,6 +82,11 @@ public DocIdSet getDocIdSet(IndexReader reader) throws IOException {
             }
         }
 
+        // if no should clauses match, return null (act as min_should_match set to 1)
+        if (res == null) {
+            return null;
+        }
+
         if (notFilters != null) {
             for (int i = 0; i < notFilters.size(); i++) {
                 if (res == null) {