How to test puts in a method

gistfile1.txt

describe Putter do
    describe "#start" do
      it "should say i put" do
        output = double('output')
        putter = Putter.new
        output.should_receive(:puts).with('i put')

        putter.start
      end
    end
  end

class Putter
  def initialize
    @output = STDOUT
  end

  def start
    @output.puts "i put"
  end
end

=begin
 1) EchoBaseDirs Putter#start should say i put
     Failure/Error: output.should_receive(:puts).with('i put')
       (Double "output").puts("i put")
           expected: 1 time
           received: 0 times

rspec (2.8.0)
rspec-core (2.8.0)
rspec-expectations (2.8.0)
rspec-mocks (2.8.0)
ruby 1.9.3p0 (2011-10-30 revision 33570) [x86_64-darwin11.2.0]
=end