Cache payload in _properties
Cache payload in _properties

This bit us earlier, as we were calling:

```javascript
var probe = Probe.createProbeResponse(current),
    payload = probe.getPayload();
```

but, though `createProbeResponse` calls `Probe.getPayloadValue`, which abstracts over the attachment vs non-attachment payload possibilities, calling `probe.getPayload()` was pulling the payload directly out of `current` again, which causes problems when you think the "attachment payload" case is already covered.

This change would only pull the `payload` out of the `eccGr` if it hasn't already been stored in `_properties["payload"]`.
cjnanda/ServiceNow