Allow throttle period option to be a proc

I need to filter requests on a period I need to get dynamically out of
information I have in the requests. Currently, I can work out the limit,
as it can be a `Proc`, however I can't do that with the period.

This PR adds support for that. Tried to do it in a way that doesn't
brake backwards compatibility, as periods are coerced to numbers during
`Rack::Throttle` initialization.