Creating job with an undefined priority causes redis error

Dsing the `priority` method with an undefined value causes when creating a job causes `node_redis` to complain with:
```
node_redis: Deprecated: The HSET command contains a "undefined" argument.
This is converted to a "undefined" string now and will return an error from v.3.0 on.
Please handle this in your code to make sure everything works as you intended it to.
```

Would it not be preferable to have Kue throw an exception if the priority is undefined?
It took me some time to find the cause of this problem in my code :p

Start at the priority method and follow the job-creation path that writes to Redis. Reproduce the undefined-priority case and inspect how related invalid arguments are handled. Done means Kue raises an exception instead of passing an undefined value to node_redis, with coverage for the behavior if the repository has relevant tests.

Written by the indexing model from the issue text.