stub.as_null_object sends string interpolation into infinite loop

[xaviershay]

The following code enters an infinite loop on 2.10.1 and also 2.6 (i.e. not a regression).
I ran across this trying to render a view template with a null object, where I had a helper that was formatting a number from the object.

describe 'null objects' do
  it 'allows string interpolation' do
    "%i" % stub.as_null_object
  end
end