Modified Model.geoNear to support passing either a geoJSON point, or a legacy coordinate pair, to the native driver

support [was added](https://github.com/mongodb/node-mongodb-native/pull/1198) to the mongodb native driver for passing a geoJSON point as is to the geoNear command.  this commit adds support for that in mongoose by modifying Model.geoNear to send a legacy coordinate pair OR a geoJSON point to the driver.