Account for time zones in DateTime serializations (#102)

* Use differing time zones in list test

This proves the existence of a time zone-related bug where serializing
with time zone information breaks equality checks.

* Convert Date/DateTimes to UTC before serializing

Author: nshki

lib/kredis/type/datetime.rb

@@ -4,7 +4,7 @@ module Kredis
   module Type
     class DateTime < ActiveModel::Type::DateTime
       def serialize(value)
-        super&.iso8601(9)
+        super&.utc&.iso8601(9)
       end
 
       def cast_value(value)

test/types/list_test.rb

@@ -44,7 +44,7 @@ class ListTest < ActiveSupport::TestCase
   test "typed as datetime" do
     @list = Kredis.list "mylist", typed: :datetime
 
-    @list.append [ 1.day.from_now.midnight, 2.days.from_now.midnight ]
+    @list.append [ 1.day.from_now.midnight.in_time_zone("Pacific Time (US & Canada)"), 2.days.from_now.midnight.in_time_zone("UTC") ]
     assert_equal [ 1.day.from_now.midnight, 2.days.from_now.midnight ], @list.elements
 
     @list.remove(2.days.from_now.midnight)