Make test_helper requests that trigger an errback fail by default

The current test_helper requests have some devious default behavior.

Consider the following specs:

```ruby
describe 'sending requests' do
  let(:my_request) do
    {
      :domain => localhost:9001, # A port the server is NOT running on!!
      :path => '/some_path'
    }
  end

  context 'when sent to invalid path' do
    it 'sends a 500' do
      with_api(MyApi) do
        get_request(my_request) do |http|
          http.response_header.status.should == 500
        end
      end
    end
  end

  context 'when sent to a valid path' do
    it 'sends a 200' do
      with_api(MyApi) do
        get_request(my_request) do |http|
          http.response_header.status.should == 200
        end
      end
    end
  end
end
```

Both of these specs pass! As soon as you add the optional errback param, as a proc that raises, the true failures show through on both specs.