force set default document category if it is not set on database (#11665)

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@10221 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/models/document_category.rb

@@ -31,4 +31,12 @@ class DocumentCategory < Enumeration
   def transfer_relations(to)
     documents.update_all("category_id = #{to.id}")
   end
+
+  def self.default
+    d = super
+    if d.nil?
+      d = find(:first)
+    end
+    d
+  end
 end