Allow to add connection with through data for HasManyThrough relation

e.g.:

```js
physician.patients.add(patient, { date: new Date(now) }, function (e, app) {
   ...
});
```

Signed-off-by: Clark Wang <clark.wangs@gmail.com>
hasAndBelongsTo has an implicit through model, so I think it's best to avoid additional properties there (mongodb will accept it anyway, but other datasources probably wouldn't). So the check over *loopback* should be more specific (not just test for relation.modelThrough).