title: Implicit grant Laravel 5.4 passport unsupported_grant_type error
body: <p>I have successfully implemented authorization code grant and password grant with passport 2.0 and laravel 5.4. After adding the Passport::enableImplicitGrant(); in AuthServiceProvider.php, I tried implementing the implicit grant with an angular2 app.</p>
  
  <pre><code>  getImplicitAccessToken() {
      const headers = new Headers({
        'Content-Type': 'application/json',
        'Accept' : 'application/json'
      });
      const query = {
        'grant_type' : 'token',
        'client_id' : Constants.IMPLICIT_TEST_CLIENT_ID,
        'redirect_uri' : window.location.origin + '/implicit-code-grant',
        'scope': ''
      };
      const params = this.getParamsFromJson(query);
      window.location.href = Constants.OAUTH_AUTHORIZATION_URL + '?' + params.toString();
    }
    private getParamsFromJson(query: any) {
      const params = new URLSearchParams();
      for (const key in query) {
        params.set(key, query[key]);
      }
      return params;
    }
  </code></pre>
  
  <p>However I get an unsupported_grant_type error</p>