Fix an express deprecation

This message was given when rate limiting is hit:

```
express deprecated res.json(status, obj): Use res.status(status).json(obj) instead node_modules/express-rate/lib/rate.js:61:17
```

The deprecation is mentioned in https://github.com/strongloop/express/releases/tag/4.7.0

Also update the docs and example.
Merge remote-tracking branch 'origin/remove-express-4.7.0-deprecated-call' into unit-test-fixup
Move handler-specific variable into handler object.

So the common stuff doesn't look like it has exceptions just for
Redis.