Always require a client for running a worker

[fkorotkov]

Otherwise it's getting overriden by the default one

[edigaryev]

Otherwise it's getting overriden by the default one

Wouldn't it be easier to fix it by just checking if the worker.client is not nil in the Worker's New(), before instantiating a default client?

This way it'd be still possible to instantiate a Worker without additionally instantiating a Client.

[fkorotkov]

I though about checking worker.client for nil but IMO explicitly requiring it for creation made me think about it. Otherwise I can see just some options and not pay too much attention to them. But the client is a crucial part of the API and it's not optional. I'd prefer to leave it like that to stress the client being presented.