BUGFIX: Use multi_match instead of query_string

By using ``multi_match`` instead of ``query_string`` within our search
query, we prevent the accidental injection of Lucene search query
strings. Currently an exception is thrown when adding ``"`` to your search query.

Using ``multi_match`` instead should lead to the same quality of results
and is less prone to user errors, because in 99% of cases the search is
used for classic search terms and no end user is expected to know the
compact Lucene query string syntax.

see: https://www.elastic.co/guide/en/elasticsearch/reference/5.6/query-dsl-multi-match-query.html
see: https://www.elastic.co/guide/en/elasticsearch/reference/5.6/query-dsl-query-string-query.html
see: https://www.elastic.co/guide/en/elasticsearch/reference/5.6/query-dsl-query-string-query.html#query-string-syntax

Author: gerhard-boden

Classes/Driver/Version5/Query/FilteredQuery.php

@@ -60,7 +60,7 @@ public function from(int $size): void
     public function fulltext(string $searchWord, array $options = []): void
     {
         $this->appendAtPath('query.bool.must', [
-            'query_string' => array_merge($options, [
+            'multi_match' => array_merge($options, [
                 'query' => $searchWord,
                 'fields' => ['__fulltext*']
             ])