Fix preparation of trailing positional operator in query field

See: http://stackoverflow.com/questions/17886359/update-embedded-subdocument-in-array-with-doctrine-and-mongodb

If `DocumentPersister::prepareQueryElement()` receives a field name ending in `$` (i.e. [positional operator](http://docs.mongodb.org/manual/reference/operator/positional/)), we should not assume that there is another field part after it. This addresses the case where an entire array element is being matched or updated.