[9.x] Fix: prevent duplicated content-type on HTTP client

[danilopolani]

The PR fixes a weird behaviour where calling multiple times a method that sets a content type (or manually) will append the same content type duplicated, eventually causing issues with external APIs (especially with Express middlewares etc.).

Sample to reproduce the old behaviour:

use Illuminate\Support\Facades\Http;

$client = Illuminate\Support\Facades\Http::asJson();

$client->asJson();
$client->asJson();
dd($client->getOptions()['headers']['Content-Type']);

Output