fixed #9542 sqlite3 exception in db:migrate on AddChangesetCommitDate

git-svn-id: http://redmine.rubyforge.org/svn/trunk@383 e93f8b46-1217-0410-a6f0-8f06a7374b81

db/migrate/036_add_changeset_commit_date.rb

@@ -1,6 +1,6 @@
 class AddChangesetCommitDate < ActiveRecord::Migration
   def self.up
-    add_column :changesets, :commit_date, :date, :null => false
+    add_column :changesets, :commit_date, :date
     Changeset.update_all "commit_date = committed_on"
   end