MyWorker.perform_in(1.month) does not always schedule job in one month.

At the moment, `MyWorker.perform_in(1.month)` always schedules a job in 30
days. When added to a date, `1.month` will add 1 to the date's month count,
which means that it will add 28, 29, 30, or 31 days depending on the month and
year.
When I call `MyWorker.perform_in(1.month)`, I would expect the job to be
scheduled next month, same day of the month, all the time. At the moment, it is
true only four months in the year.
My pull request tries to fix this by converting the interval object to a Float
at the last possible moment.
Plaese note that the test I wrote will fail only during months that do not have
30 days. Ideally, I would add a dependency to Timecop and freeze time to any
day in a month of 28, 29 or 31 days. This could also avoid using
`#assert_in_delta`, in favour of `#assert_equal`.

Feel free to read my blog post [Rails' `1.month` has a variable
length](http://dstosik.github.io/rails/2015/02/19/rails-1month-variable-length/)
for more details.

lib/sidekiq/worker.rb

@@ -42,13 +42,13 @@ def perform_async(*args)
 
       def perform_in(interval, *args)
         int = interval.to_f
-        now = Time.now.to_f
-        ts = (int < 1_000_000_000 ? now + int : int)
+        now = Time.now
+        ts = (int < 1_000_000_000 ? (now + interval).to_f : int)
 
         item = { 'class' => self, 'args' => args, 'at' => ts }
 
         # Optimization to enqueue something now that is scheduled to go out now or in the past
-        item.delete('at'.freeze) if ts <= now
+        item.delete('at'.freeze) if ts <= now.to_f
 
         client_push(item)
       end

test/test_scheduling.rb

@@ -30,6 +30,15 @@ def perform(x)
       @redis.verify
     end
 
+    it 'schedules a job in one month' do
+      @redis.expect :zadd, true do |key, args|
+        assert_equal 'schedule', key
+        assert_in_delta 1.month.since.to_f, args[0][0].to_f, 1
+      end
+      assert ScheduledWorker.perform_in(1.month, 'mike')
+      @redis.verify
+    end
+
     it 'schedules a job via timestamp' do
       @redis.expect :zadd, true, ['schedule', Array]
       assert ScheduledWorker.perform_in(5.days.from_now, 'mike')