Only merge scopes with zero arity in has_many through

with dynamic conditions.

Fixes #16128

This bug was introduced in https://github.com/rails/rails/commit/c35e438620f2d56562251571377995359546393d
so it's present from 4.1.2-rc1 and after.

https://github.com/rails/rails/commit/c35e438620f2d56562251571377995359546393d
merges any relation scopes passed as proc objects to the relation,
but does *not* take into account the arity of the lambda.

To reproduce: https://gist.github.com/Agis-/5f1f0d664d2cd08dfb9b
Merge pull request #16586 from seuros/master

Fix typo in rdoc [ci skip]
Merge pull request #16172 from Agis-/hmt_scope_arity

has_many :through with dynamic condition merging
[Rdoc] Make clear that we are sending an AR object [ci skip]