This pull requests addresses ORA-01400 errors and also supports rails…

…#6115 issue tested.

Issue rails#6115 has been fixed and tested with
the attribute `:null => false, :default => ""`
However `:null => false` attribute is not necessary to test this issue,
which causes many ORA-01400 errors with Oracle enhanced adapter.

activerecord/test/schema/schema.rb

@@ -178,7 +178,7 @@ def create_table(*args, &block)
     t.integer :client_of
     t.integer :rating, :default => 1
     t.integer :account_id
-    t.string :description, :null => false, :default => ""
+    t.string :description, :default => ""
   end
 
   add_index :companies, [:firm_id, :type, :rating, :ruby_type], :name => "company_index"