JoinDependency#build_with_polymorphism possibly works again

Looking at both +initialize+ and +build+ methods diff between Rails
4.0.2 and 4.1.0.beta1 should give some reasoning to the changes here.

It looks like all logic previously placed in JoinDependecy#build was
moved into some sort of tree so that all JoinDependecy#build does now is
find the reflection and build the JoinAssociation for that. Bad side
effect here is that JoinDependency#build_with_polymorphism overrides
rails ActiveRecord::JoinDependency#build completely.

https://github.com/rails/rails/blob/v4.0.2/activerecord/lib/active_record/associations/join_dependency.rb#L31-40
https://github.com/rails/rails/blob/v4.0.2/activerecord/lib/active_record/associations/join_dependency.rb#L128-154

https://github.com/rails/rails/blob/v4.1.0.beta1/activerecord/lib/active_record/associations/join_dependency.rb#L95-101
https://github.com/rails/rails/blob/v4.1.0.beta1/activerecord/lib/active_record/associations/join_dependency.rb#L214-225
Drop join_type examples in join_dependecy_spec

The accessor has been deprecated in rails core