app.rb

$rest = RestClient.new('http://foo.org')

get '/' do
  $rest.put '/foo'
  'hey'
end

app_spec.rb

describe 'My App' do
  before(:each) do
    @rest = mock('rest client')
    RestClient.stub!(:new).and_return(@rest)
  end

  it 'should init restc client' do
    RestClient.should_receive(:new).and_return(@rest)
  end

  it 'should do a PUT to "/foo"' do
    $rest.should_receive(:put).with('foo')
    get_it '/'
  end
end