Some fixes in IdentityMap plugin

[funny-falcon]

First commit fixes check for primary key - currently association_reflection[:primary_key] is not filled by default but lazy filled in method, so that check failed (at least in rails development mode).

Second commit fixes issue that model which had not called plugin :identity_map does not respect identity map of associated object. I don't really know, is including methods streight to Sequel::Model a best solution? but it just works.

[jeremyevans]

I think the primary key checking (8d3bd27) is correct, but I object to the plugin modifying Sequel::Model itself (36218f3). See my comment on that commit. I'll probably cherry-pick 8d3bd27.

[funny-falcon]

I made a comment there