Changed QueryBuilder method from ->where() to ->andWhere() ...

...so it works with customised QueryBuilders which have an existing where clause (instead of over-writing any existing DQL 'where' part).

Before this change if you create a customised querybuilder (e.g. through the `persistence.elastica_to_model_transformer.query_builder_method` config setting) and that QB contained a where clause then Elastica would over-write it causing a QueryException along the lines of "Invalid parameter number: number of bound variables does not match number of tokens".