SearchDescriptor, Preference exception

[jjherbst]

When specifying a preference on the query descriptor, it causes elasticsearch to throw an exception.

It seems that the preference is added to the body part of the query instead of the search parameters.

ElasticSeacrh code to re-produce

var result = elasticClient.Search<OrganizationProfile>(
    op=>op.Query(
            q=>q.Term(
                    t=>t.Profile.PersonalDetail.FirstName,"juan"
            )
    )
    .ExecuteOnPrimary()
    .Fields(field => field.Profile)
    .From(0)
    .Size(2)
);

Fiddler output (sent to ES):
{
"from": 0,
"size": 2,
"query": {
"term": {
"Profile.PersonalDetail.FirstName": {
"value": "juan"
}
}
},
"fields": [
"Profile"
],
"preference": "_primary"
}

Exception:
Parse Failure [No parser for element [preference]]]; }]