Model#import fails to find model's primary key sequence

Fixes a bug where Model#import does not work when a model's primary key
has no associated 'sequence' in the database. If a value for the primary
key is specified, then the insert should not require a database
'sequence' to generate a value for that primary key field.

test/schema/generic_schema.rb

@@ -95,4 +95,8 @@ ActiveRecord::Schema.define do
   end
   
   add_index :animals, [:name], :unique => true, :name => 'uk_animals'
+
+  create_table :widgets, :id => false, :force => true do |t|
+    t.integer :w_id
+  end
 end