queue.process(name, processor) not working when using a separate process

Created by: sylvainlap

Hello,

In the reference guide (https://github.com/OptimalBits/bull/blob/master/REFERENCE.md#queueprocess), it is written that you can define a processing function thanks to process(name, processor).

However, using this way, when the processor is a separate process (ie a string) thows a TypeError: Cannot read property 'bind' of undefined.

According to https://github.com/OptimalBits/bull/blob/master/lib/queue.js#L522, if you use this way to define your processor as a separate process, this.setHandler(name, handler); leads to an handler to undefined.