Don't eagerly load associations with :finder_sql

Has-many and has-many-through associations with :finder_sql incorrect SQL, because eager_record assumes the wrong (or even non-existing) foreign key. We avoid automatic preloading of these.

Example from the specs, without the change in lib/:

```
  1) EagerRecord has_many associations with :finder_sql should not eagerly load
     Failure/Error: Post.all.first.unapproved_commenters.should == [user, user]
     ActiveRecord::StatementInvalid:
       SQLite3::SQLException: no such column: users.post_id: SELECT "users".* FROM "users" WHERE ("users".post_id IN (1,2,3,4)) 
     # ./lib/eager_record/eager_preloading.rb:21:in `find_by_sql_with_eager_preloading'
     # ./lib/eager_record/eager_preloading.rb:76:in `load_target_with_eager_preloading'
     # ./spec/examples/eager_record_spec.rb:135:in `block (3 levels) in <top (required)>'
```