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/support/factories.rb

@@ -10,4 +10,8 @@ end
 Factory.define :topic do |m|
   m.sequence(:title){ |n| "Title #{n}"}
   m.sequence(:author_name){ |n| "Author #{n}"}
-end
+end
+
+Factory.define :widget do |m|
+  m.sequence(:w_id){ |n| n}
+end