How can I send JSON When using FeatureTestCase?

API Document example

Create member

Code:

POST /members HTTP/1.1
Host: localhost
Content-Type: application/json

{
    "name" : "mike",
    "age" : 30
}

MemberController.php

PHP Code:

<?php
class MemberController extends Controller
{
    function create()
    {
        $data = $this->reqest->getJSON();  // <-- parse JSON
        // codes..
    }
} 


In this case, I want to make test function like this

PHP Code:

<?php
class MemberTest extends FeatureTestCase
{
    public function testCreateMember() {
        $body = [
            'name' => 'mike',
            'age' => 30,
        ];
        $result = $this->post('member/v2/organs', $body);
        // codes..
    }
} 


It doesn't work.
How can I send JSON when FeatureTestCase ?