Stubs for Typhoeus::Request#on_body

Typhoeus 0.6.7 includes support for streaming. This branch adds support for mocking streamed typhoeus requests. For example, it makes it possible to use wemock to do the stubbing for this code:

```ruby
request = Typhoeus::Request.new('https://github.com')
request.on_body { |chunk| p chunk }
request.run
```

cc typhoeus/typhoeus#339