Add json helper method to set payload as json-encoded value.

[mjauvin]

Allows doing this:

$payload = [
    'key1' => 'value1',
    'key2' => 'value2'
];

$apiResult = Http::post($url, function ($http) use ($payload) {
    $http->json( $payload );
});

Instead of this:

$payload = [
    'key1' => 'value1',
    'key2' => 'value2'
];

$apiResult = Http::post($url, function ($http) use ($payload) {
    $http->header('Content-Type', 'application/json; charset=utf-8');
    $http->setOption( CURLOPT_POSTFIELDS, json_encode($payload) );

[mjauvin]

Any objection to merge this one ?