Forum: RSpec Problem with mocking helper methods

Given that I assume that rails helper image_tag works as intended, I
want  to mock out call to this method in my helper.

spec/helpers/some_helper_spec.rb:
require 'spec_helper'
describe SomeHelper do
  it  'allows stub tags' do
       helper.should_receive(:image_tag).with(2).and_return(3)
       helper.image_tag(2).should == 3
  end
end

result:
       Failure/Error : helper.image_tag(2).should == 3
       expected : 3
       got : "<img alt=\"2\" src=\"/assets/2\" />" (using ==)

Bundle:
 * rspec (2.7.0)
 * rspec-core (2.7.1)
 * rspec-expectations (2.7.0)
 * rspec-mocks (2.7.0)
 * rspec-rails (2.7.0)
 * rails (3.1.1)