Use string datatype for the setting attribute

to make store works all database adapters.

activerecord/test/schema/schema.rb

@@ -37,7 +37,7 @@ ActiveRecord::Schema.define do
 
   create_table :admin_users, :force => true do |t|
     t.string :name
-    t.text :settings, :null => true
+    t.string :settings, :null => true, :limit => 1024
     # MySQL does not allow default values for blobs. Fake it out with a
     # big varchar below.
     t.string :preferences, :null => false, :default => '', :limit => 1024