Calling `poll_interval=` doesn't account for scaling due to interval randomization

While integrating Sidetiq and needing to adjust the scheduled job polling frequency, I discovered this issue.

Workaround is included here for more information and context: https://github.com/tobiassvn/sidetiq/wiki/Configuration#polling 

It seems to me that it's the `poll` methods responsibility to account for the randomization scaling, not the `poll_interval` method's. If you'd like it done a different way, or if I'm thinking incorrectly about this being an issue, just let me know.