any_instance.stub

describe "any_instance.stub" do
  it "returns the specified value on any instance of the class" do
    Object.any_instance.stub(:foo).and_return(:return_value)

    o = Object.new
    o.foo.should eq(:return_value)
  end
end

https://www.relishapp.com/rspec/rspec-mocks/v/2-6/docs/method-stubs/stub-on-any-instance-of-a-class