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>
@clarkorz the referencesMany relation should not take additional data. So it's best to check for this over at *loopback*'s setup of remoting, and skip it there.