[7.x] Add HTTP client response body as object

[nuernbergerA]

This PR lets you access the Http client response body as an object.

I also added tests to cover existing ways to access the response body.

Current state

// Current magic way - uses ->json() under the hood
$response = Http::get('some-api.wip');
$response['result'];

// Or with json()
$response = Http::get('some-api.wip')->json();
$response['result'];

Initially, I thought to pass a boolean to json() to tell if it should convert JSON as array or object.

But the fact that json() is used to implement ArrayAccess, and json(true) isn't verbose at all, I went with object()

Usage

// New option
$response = Http::get('some-api.wip')->object();
$response->result;

Future

As most API's wrap there responses with data or result.

// I thought it would be nice to do this
$response = Http::get('some-api.wip')->object('result');

But didn't implement it yet to keep it simple for now.

I hope you find it useful.
Happy Easter to everyone.

[mreduar]

Great! Thanks for this!