Ensure run_at is a string when sorting by it.

When using Resque and Sidekiq at the same time, resque-web errors out if there are any Sidekiq workers processing jobs. It seems it is because "run_at" in Resque is an iso8601 string, whereas in Sidekiq it's a number. As such, #to_s the run_at to ensure that we're dealing with a string (especially since the nil case compares against empty string).