specifying output of render call

Hello again,

First of all, thanks much for everyone's input on my question last week about spec'ing an invocation of super.  

Here's another question that seems resistant to my Google searching:

With this helper method (defined in a module and included in Rails controllers):

def render_rjs_redirect(url = '/')
  render :update do |page|
    page << "location.href='#{url}'"
  end
end

Is it possible/advisable to specify either the block passed to render or the output of the helper, something like below?  The only way I have been able so far to spec this was to create a FooController with an index action that called the method and then checked the response.body, but that seems like a lot of overhead to test such a simple method.  Is there a better or obvious way to do this?

Thanks!
Matt

it "should render javascript to redirect the browser" do
  self.should_receive(:render).with(:update) { |page| page << "location.href='example.com'"}
  render_rjs_redirect('example.com')
end

it "should render javascript to redirect the browser" do
  render_rjs_redirect('example.com')
  response.body.should == "location.href='example.com'"
end