Problem when adding multiple requests in a callback

```ruby
hydra = Typhoeus::Hydra.hydra

first_request = Typhoeus::Request.new("www.example.com/posts/1.json")
first_request.on_complete do |response|
  second_request = Typhoeus::Request.new("www.example.com/posts/2.json")
  third_request = Typhoeus::Request.new("www.example.com/posts/3.json")
  hydra.queue second_request  
  hydra.queue third_request
end

hydra.queue first_request
hydra.run
```
In this example I expect second_request and third_request to execute in parallel, but actually they execute in sequence. :disappointed:

This PR is a try to fix that.
I wasn't able to come up with a test for this special case. 
Maybe someone can help with that.