Use JSON date serializer

Effective API changes:

  - nil serializes to null, not empty string
  - dates serialize to IETF RFC 3339 with millisecond precision, not
    second precision surrounded by spaces

app/serializers/event_schedule_serializer.rb

@@ -3,12 +3,8 @@
 class EventScheduleSerializer < ActiveModel::Serializer
   include ActionView::Helpers::TextHelper
 
-  attributes :date, :room
-
-  def date
-    t = object.start_time
-    t.blank? ? '' : %( #{I18n.l t, format: :short}#{t.formatted_offset(false)} )
-  end
+  attribute :start_time, key: :date
+  attributes :room
 
   def room
     object.room.guid

app/serializers/event_serializer.rb

@@ -3,12 +3,9 @@
 class EventSerializer < ActiveModel::Serializer
   include ActionView::Helpers::TextHelper
 
-  attributes :guid, :title, :length, :scheduled_date, :language, :abstract, :speaker_ids, :type, :room, :track
-
-  def scheduled_date
-    t = object.time
-    t.blank? ? '' : %( #{I18n.l t, format: :short}#{t.formatted_offset(false)} )
-  end
+  attributes :guid, :title, :length
+  attribute :time, key: :scheduled_date
+  attributes :language, :abstract, :speaker_ids, :type, :room, :track
 
   def speaker_ids
     speakers = object.event_users.select { |i| i.event_role == 'speaker' }

spec/serializers/event_schedule_serializer_spec.rb

@@ -12,7 +12,7 @@ describe EventScheduleSerializer, type: :serializer do
 
   it 'sets date and room' do
     expected_json = {
-      date: ' 2000-01-02T03:04:05+0000 ',
+      date: '2000-01-02T03:04:05.000Z',
       room: event_schedule.room.guid
     }.to_json
 

spec/serializers/event_serializer_spec.rb

@@ -11,7 +11,7 @@ describe EventSerializer, type: :serializer do
         guid:           event.guid,
         title:          'Some Talk',
         length:         30,
-        scheduled_date: '',
+        scheduled_date: nil,
         language:       nil,
         abstract:       'Lorem ipsum dolor sit amet',
         speaker_ids:    event.speaker_ids,
@@ -41,7 +41,7 @@ describe EventSerializer, type: :serializer do
         guid:           event.guid,
         title:          'Some Talk',
         length:         30,
-        scheduled_date: ' 2014-03-04T09:00:00+0000 ',
+        scheduled_date: '2014-03-04T09:00:00.000Z',
         language:       'English',
         abstract:       'Lorem ipsum dolor sit amet',
         speaker_ids:    [speaker.id],