Description: add missing require 'date' statement
 The file lib/prawn/table/cell.rb uses the test .kind_of?(Date) but the class
 Date is not defined.
Bug: https://github.com/sandal/prawn/issues/298
Author: Cédric Boutillier <cedric.boutillier@gmail.com>
Last-Update: 2011-11-22

--- a/lib/prawn/table/cell.rb
+++ b/lib/prawn/table/cell.rb
@@ -6,6 +6,8 @@
 #
 # This is free software. Please see the LICENSE and COPYING files for details.
 
+require "date"
+
 module Prawn
   class Document