* Call block to #redirect_to in controller context

The documentation for ActionController::Redirecting states that a Proc
argument "will be executed in the controller's context."  However,
unless #instance_eval is used (removed in 6b3ad0ca), that statement is
false for procs defined outside of the controller instance.

This commit restores the documented behavior.

Fixes #33731.

* Move test proc into a constant in another class

Per @rafaelfranca's suggestion.

[Steven Peckins + Rafael Mendonça França]