Ensure that the cut-off time is in the same timezone as the extended …

…object

If we don't match the UTC offset, and the system timezone is not UTC, you end up
with the following scenario:

= Scenario 1 (correct)
Time.now               # 2014-08-15 15:30:00 +0100
Time.now.cut_off_time  # 2014-08-15 16:00:00 +0100
Time.now.despatch_day  # Fri, 15 Aug 2014

= Scenario 2 (incorrect)
Time.zone.now               # Fri, 15 Aug 2014 15:30:00 BST +01:00
Time.zone.now.cut_off_time  # Fri, 15 Aug 2014 16:00:00 BST +01:00
Time.zone.now.despatch_day  # Mon, 18 Aug 2014

The incorrect result in Scenario 2 is due to the fact that the
ActiveSupport::TimeWithZone class wraps a UTC time object and maintains the
timezone data separately. When DateExtension.despatch_day is called in Scenario
2, the context is:

self          # 2014-08-15 15:30:00 UTC
cut_off_time  # 2014-08-15 16:00:00 +0100

This comparison is incorrect due to the mismatch in timezones and produces the
wrong result. The fix is to construct the cut_off_time based on the timezone
of #self.

lib/nextday/date_extension.rb

@@ -30,7 +30,8 @@ def delivery_day
     def cut_off_time
       hour, minute = Config.cut_off_hour, Config.cut_off_minute
 
-      Time.new(year, month, day, hour, minute)
+      utc_offset = self.utc_offset  unless not self.respond_to?(:utc_offset)
+      Time.new(year, month, day, hour, minute, 0, utc_offset)
     end
 
     ##