[Fix #1128] Make Rails/DuplicateAssociation aware of duplicate class_name

[koic]

Fixes #1128.

This PR makes Rails/DuplicateAssociation aware of duplicate class_name.

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[franzliedke]

@koic This example (with belongs_to) is a false positive. Here's an example that should make this clear:

class GraphEdge
  belongs_to :source_node, class_name: "GraphNode"
  belongs_to :target_node, class_name: "GraphNode"
end

Contrary to has_many, the names of belongs_to associations are not arbitrary, as they implicitly reference the foreign key.

[bquorning]

I noticed the same problem after upgrading to v2.22.0 today.

I think the “identical class_name value” is a valid offense for both :has_one, :has_many and :has_and_belongs_to_many – but not for :belongs_to.