Remove User#at_midnight

Replace it with a class called DateUtils with a method called
`#midnight_for` that takes a preference instance and date to convert to
midnight.

test/controllers/recurring_todos_controller_test.rb

@@ -117,7 +117,9 @@ class RecurringTodosControllerTest < ActionController::TestCase
     new_todo = Todo.where(:recurring_todo_id => 5).first
 
     # due date should be the target_date
-    assert_equal users(:admin_user).at_midnight(Date.new(target_date.year, target_date.month, target_date.day)), new_todo.due
+    user = users(:admin_user)
+    target_date = Date.new(target_date.year, target_date.month, target_date.day)
+    assert_equal DateUtils.midnight_for(user.prefs, target_date), new_todo.due
 
     # show_from should be nil since now+4.days-10.days is in the past
     assert_equal nil, new_todo.show_from

test/models/preference_test.rb

@@ -20,7 +20,8 @@ class PreferenceTest < ActiveSupport::TestCase
   end
   
   def test_parse_date
-    assert_equal @admin_user.at_midnight(Date.new(2007, 5, 20)).to_s, @admin_user.preference.parse_date('20/5/2007').to_s
+    date = Time.new(2007, 05, 20).in_time_zone(@admin_user.preference.time_zone).at_midnight
+    assert_equal date.to_s, @admin_user.preference.parse_date('20/5/2007').to_s
   end
   
   def test_parse_date_returns_nil_if_string_is_empty