Remove the host header when cleaning headers to properly follow the redirection.

Hello @visionmedia,

The redirection is not followed correctly when the address given by the  `location` header  contained a new host compared to the previous request. 

For example this code fails to fetch the google main page:

```js
var redirections = [];
request
  .get('http://google.com')
  .on('redirect',function(res){
    redirections.push(res.headers.location);
  })
  .redirects(10)
  .end(function(err,res){
     console.log(res.status); // 301
     console.log(redirections); 
    // always the same url -> http://www.google.com/ because it requests only this page http://google.com
  });
```

The problem is fixed by removing the `host` header in the `cleanHeader` method in `utils`.
This old `host` header overrides the new one there https://github.com/visionmedia/superagent/blob/master/lib/node/index.js#L477.