[5.4] Properly set the JSON payload of FormRequests

[themsaid]

This PR highlights the cause of the issue experienced in #17753

When the transformation middleware runs, $request->json() is called and the returned parameter bag is replaced with the transformed one, later calls to ->json() presents the modified bag not the original one: https://github.com/laravel/framework/blob/5.4/src/Illuminate/Http/Request.php#L294

For form requests when we initialise it we pass the original content:

framework/src/Illuminate/Foundation/Providers/FormRequestServiceProvider.php

Line 56 in 7212b1e

$current->cookies->all(), $files, $current->server->all(), $current->getContent()

So when ->json() is called on the form request, it'll read the original content of the request and thus why the transformation won't take effect.

In this PR I explicitly set the json bag of the form request from the json bag of the original request, this way ->json() won't have to look into the content since there's already a bag present.