Fixed: not null constraints not removed with Postgresql.

git-svn-id: http://redmine.rubyforge.org/svn/trunk@1140 e93f8b46-1217-0410-a6f0-8f06a7374b81

db/migrate/048_allow_null_version_effective_date.rb

@@ -1,6 +1,6 @@
 class AllowNullVersionEffectiveDate < ActiveRecord::Migration
   def self.up
-    change_column :versions, :effective_date, :date, :default => nil
+    change_column :versions, :effective_date, :date, :default => nil, :null => true
   end
 
   def self.down