boolean_query jsonSerialize uses wrong property name for must_not

boolean_query jsonSerialize uses wrong property name for must_not.

The propery named updated by mustNot() is "mustNot", while the property name used to serialize is "must_not".

This results in 

Fatal error: Uncaught Couchbase\BaseException: LCB_ERR_PARSING_FAILURE (208)

The following will demonstrate the problem - the serialized bool_query is an empty array.

    $interval_query = (new \Couchbase\DateRangeSearchQuery())

       ->start(0)

       ->end(0)

       ->field('deleted_at');

    $disjunction_query = new \Couchbase\DisjunctionSearchQuery([$interval_query]);

    $bool_query = new \Couchbase\BooleanSearchQuery();

    $bool_query->mustNot($disjunction_query);

    //var_dump($disjunction_query->jsonSerialize());

    var_dump($bool_query->jsonSerialize());