[8.x] Enable to modify HTTP Client request headers when using beforeSending() callback

[ianriizky]

Basically, I'm trying to add a request header when sending an HTTP request using HTTP Client from Laravel. But I want to make this thing dynamically since it requires the targeted URL as the header data and it will be used in many further HTTP request. So I decide to use the Http::beforeSending() method and take the given Illuminate\Http\Client\Request parameter to get the URL as the example shown below.

<?PHP

use Illuminate\Http\Client\PendingRequest;
use Illuminate\Http\Client\Request;
use Illuminate\Support\Facades\Http;

Http::beforeSending(function (Request $request, array $options, PendingRequest $pendingRequest) {
    $requestLine = sprintf(
        '%s %s HTTP/%s',
        $request->toPsrRequest()->getMethod(),
        $request->toPsrRequest()->getUri()->withScheme('')->withHost(''),
        $request->toPsrRequest()->getProtocolVersion()
    );

    // this way doesn't work as I mention below.
    $pendingRequest->withHeaders(['Authorization' => 'Bearer '.$requestLine]);

    // this way works, but need some changes in the HTTP client.
    return $request->toPsrRequest()->withHeader('Authorization', 'Bearer '.$requestLine);
})->get('http://foo.com/json');

The problem is the request header wasn't changed at all since the PendingRequest::runBeforeSendingCallbacks() used to run this callback doesn't bring that into the real HTTP request as I expect. Using PendingRequest::withHeaders() also doesn't work because the actual HTTP Request was already created, so any modification that comes from the PendingRequest instance is useless.

This PR will fix that problem, so every registered beforeSending callback on the HTTP Client that returns a GuzzleHttp\Psr7\RequestInterface or Illuminate\Http\Client\Request value will be applied to the HTTP request instance.