new http-headers-host option

Hi Andrea,
I think the newest version 0.2.5 has broken the google-search.js test. The host header property is the problem.
![screen shot 2014-02-27 at 22 15 51](https://f.cloud.github.com/assets/879612/2288656/dca2f374-9ffc-11e3-89f7-cad53a5315ca.png) 
I think you could consider to use it as custom option and not to set it by default having the same value of the host option.