Useful for computing custom headers or resolving relative URLs.

Here's an example how how it can be used to support relative URLs and adding a Hawk Authentication header:

Request::ini(Request::init()
             ->followRedirects(true)
             ->sendsAndExpects('application/json')
             ->withAccept('application/json')
             ->beforeSend(function($req) use ($onslip360_url, $onslip360_key) {
                     // Resolve relative URLs
                     $req->uri($onslip360_url->resolve($req->uri)->getURL());

                     // Add Authentication header
                     $hawk = \Dflydev\Hawk\Client\ClientBuilder::create()
                         ->build()
                         ->createRequest($onslip360_key, $req->uri, $req->method,
                                         !isset($req->serialized_payload) ? array() : array('payload'      => $req->serialized_payload,
                                                                                            'content_type' => $req->content_type));
                     $req->addHeader($hawk->header()->fieldName(), $hawk->header()->fieldValue());
                 }));