prevent OnCompleteFunc called before last retry

[hurngchunlee]

Hi @thoas,

First of all, thank you for creating this useful library for task scheduling.

I noticed that the queue.OnCompleteFunc is triggered before the last retry takes place. After checking the source code, I think the changes in this pull request will prevent it to happen.

Cheers, Hong

[hurngchunlee]

@thoas sorry for taking so long to get back to the failure of the test, was busy on other projects. The failure was due to the fact that the finished task should have the MaxRetries less than zero (or equal to -1) and therefore I update the test case in task_test.go. Could you please have another look? Thanks!