title: Guzzle form_params not accepting array
body: <p>I am using Guzzle 6 and I can't pass array with form_params in the body of the client</p>
  
  <pre><code>$postFields = [
      form_params =&gt; [
          'data[test]' =&gt; "TEST",
          'data[whatever]' =&gt; "Whatever..."
      ]
  ];
  
  $client = new GuzzleClient([
          'cookies' =&gt; $jar, // The cookie
          'allow_redirects' =&gt; true, // Max 5 Redirects
          'base_uri' =&gt; $this-&gt;navigateUrl, // Base Uri
          'headers' =&gt; $this-&gt;headers
  ]);
  $response = $client-&gt;post('api',[$postFields]);
  </code></pre>
  
  <p>Finally, when i send the request my data is gone... But if I manually add the the data in the response it's working fine. </p>
  
  <pre><code>$response = $client-&gt;post(
      'api',
      [form_params =&gt; [
          'data[test]'=&gt;"TEST",
          'data[wht]' =&gt; 'Whatever'
      ],
  ]
  // It's working this way...
  </code></pre>
  
  <p>I hope I am clear enough if u need more info feel free to ask. Thanks in advance.</p>