undefined method `preloaded_records' for :relation:Symbol

In rails 4.1.4, using squeel@master to eager load a habtm or `has_many through:` results in an error from deep inside the ActiveRecord bowels: ` undefined method 'preloaded_records' for :tag:Symbol`.  Looking at adapters/active_record/4.1/preloader_extensions.rb, it seems that preload_with_squeel returns something (the preload keypath as a nested hash) that is nothing like preload_without_squeel (an array of ActiveRecord::Associations::Preloader::BelongsTo or similar).

This PR currently adds two simple failing specs to the squeel suite (at least when run with "RAILS=v4.1.4 AREL=5.0.1").  I will try to dig deeper, but not sure I have the mojo to figure out all of the moving parts.

Full trace from the error:
```
     Failure/Error: relation = Article.includes{tags}.to_a
     NoMethodError:
       undefined method `preloaded_records' for :tag:Symbol
     # activerecord/lib/active_record/associations/preloader/through_association.rb:32:in `each'
     # activerecord/lib/active_record/associations/preloader/through_association.rb:32:in `flat_map'
     # activerecord/lib/active_record/associations/preloader/through_association.rb:32:in `associated_records_by_owner'
     # activerecord/lib/active_record/associations/preloader/has_many_through.rb:8:in `associated_records_by_owner'
     # activerecord/lib/active_record/associations/preloader/collection_association.rb:13:in `preload'
     # activerecord/lib/active_record/associations/preloader/association.rb:20:in `run'
     # activerecord/lib/active_record/associations/preloader.rb:137:in `block (2 levels) in preloaders_for_one'
     # activerecord/lib/active_record/associations/preloader.rb:135:in `each'
     # activerecord/lib/active_record/associations/preloader.rb:135:in `map'
     # activerecord/lib/active_record/associations/preloader.rb:135:in `block in preloaders_for_one'
     # activerecord/lib/active_record/associations/preloader.rb:134:in `each'
     # activerecord/lib/active_record/associations/preloader.rb:134:in `flat_map'
     # activerecord/lib/active_record/associations/preloader.rb:134:in `preloaders_for_one'
     # activerecord/lib/active_record/associations/preloader.rb:106:in `preloaders_on'
     # ./lib/squeel/adapters/active_record/4.1/preloader_extensions.rb:21:in `block in preload_with_squeel'
     # ./lib/squeel/adapters/active_record/4.1/preloader_extensions.rb:20:in `each'
     # ./lib/squeel/adapters/active_record/4.1/preloader_extensions.rb:20:in `preload_with_squeel'
     # activerecord/lib/active_record/relation.rb:610:in `block in exec_queries'
     # activerecord/lib/active_record/relation.rb:609:in `each'
     # activerecord/lib/active_record/relation.rb:609:in `exec_queries'
     # activerecord/lib/active_record/relation.rb:486:in `load'
     # activerecord/lib/active_record/relation.rb:231:in `to_a'
     # ./spec/squeel/adapters/active_record/relation_extensions_spec.rb:343:in `block (3 levels) in <module:ActiveRecord>'
```