title: Laravel Passport Client Credentials Grant
body: <p>I use Laravel Passport for my API authentication. I have already set up a Password Credentials Grant and it works. Now I need a Client Credentials Grant for machine-to-machine authentication.
  <br>I created a new client with 
  <code>php artisan passport:client --client</code>.
  I tried to make a request to /oauth/token with this body (with Insomnia):</p>
  
  <pre><code>{
      "client_id":3,
      "secret":"wJWuVVydkHIQQ6gC7xvd0eEKytIFAD3pa149e6TR",
      "grant_type":"client_credentials"
  }
  </code></pre>
  
  <p>That's the response i get from Passport:</p>
  
  <pre><code>{
    "error": "invalid_client",
    "error_description": "Client authentication failed",
    "message": "Client authentication failed"
  }
  </code></pre>
  
  <p>Does anyone have any idea why it is an invalid client?<br>
  This is the entry in the oauth_clients table for my client, exported as json:</p>
  
  <pre><code>{
      "id":"3",
      "user_id":null,
      "name":"ClientCredentials Grant Client",
      "secret":"wJWuVVydkHIQQ6gC7xvd0eEKytIFAD3pa149e6TR",
      "redirect":"",
      "personal_access_client":"0",
      "password_client":"0",
      "revoked":"0",
      "created_at":"2019-04-30 17:06:17",
      "updated_at":"2019-04-30 17:06:17"
  }
  </code></pre>