fix QueryStringMultiField rewrite parameter

[samsullivan]

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