fix QueryStringMultiField rewrite parameter

With the `rewrite` parameter outside of the `query_string` model, I would receive the following error:

    [_na] query malformed, must start with start_object

Moving that parameter into the model fixed the issue.  Also, `constant_score_default` doesn't seem to be a valid value for `rewrite` anymore.  Instead, `constant_score_auto` seems like the default value.

    When not set, or set to constant_score_auto, defaults to automatically choosing either constant_score_boolean or constant_score_filter based on query characteristics.
Source: http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/query-dsl-multi-term-rewrite.html