Having some trouble testing module instance method behavior.

example_stub_problem.rb

module Foo
  def bar
    "#{to_s} whizbang"
  end
end

describe Foo do
  describe "#bar" do
    it "works" do
      base = Class.new { include Foo }
      # This allow fails saying base doesn't implement :read
      allow(base).to receive(:to_s) { "test" }
      expect(base.bar).to eq "test whizbang"
    end
  end
end