spec: add to_datetime to SortFirst and SortLast

rails3

Change-Id: I3f61de3f8302b8ab3d11ef41590ee20b5b602b19
Reviewed-on: https://gerrit.instructure.com/29560
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Florence <ryanf@instructure.com>
Product-Review: Ryan Florence <ryanf@instructure.com>
QA-Review: Ryan Florence <ryanf@instructure.com>

lib/sort_first.rb

@@ -30,6 +30,10 @@ class SortFirstClass
     nil
   end
 
+  def to_datetime
+    -Date::Infinity.new
+  end
+
   def inspect
     'SortFirst'
   end

lib/sort_last.rb

@@ -30,6 +30,10 @@ class SortLastClass
     nil
   end
 
+  def to_datetime
+    Date::Infinity.new
+  end
+
   def inspect
     'SortLast'
   end