title: Change content-type in Dio request
body: <p>Please tell me how can I change content-type of my request with Dio? I am trying to change it by adding <code>contentType: Headers.formUrlEncodedContentType</code> into options but it looks like to no avail.</p>
  <p>My code is:</p>
  <pre><code>  final String urlString4 = protocol + baseUrl + '/connect/token';
  
    var loginBody4 = FormData.fromMap({
      'code': code,
      'grant_type': 'authorization_code',
      'redirect_uri': 'courier-mobile-app://auth/login',
      'code_verifier': 'application_application_application_application_application',
      'client_id': 'courier_mobile_app',
      'client_secret': 'secret'
    });
  
    try {
      response = await dio.post(urlString4,
          data: loginBody4,
          options: Options(
            contentType: Headers.formUrlEncodedContentType
          )
      );
    } on DioError catch (e) {
      print('Request failed with status: ${e.response?.data}');
    }
  </code></pre>
  <p>but my intercepted request looks like this:</p>
  <pre><code>curl -H &quot;user-agent: Dart/2.16 (dart:io)&quot;
  -H &quot;content-type: multipart/form-data; boundary=--dio-boundary-3038081899&quot;
  -H &quot;cookie: .AspNetCore.Antiforgery.2P6h…..h3g&quot;
  -H &quot;host: auth.dev-drive.XXX.dev
  --data-binary &quot;----dio-boundary-3038081899
  content-disposition: form-data; name=\&quot;code\&quot;
  
  C8C3E5D1ACE01DF61F988E06C391AAB3347BD2A64A0B6F56E5DC225710BEF672
  ----dio-boundary-3038081899
  content-disposition: form-data; name=\&quot;grant_type\&quot;
  
  authorization_code
  ----dio-boundary-3038081899
  content-disposition: form-data; name=\&quot;redirect_uri\&quot;
  
  courier-mobile-app://auth/login
  ----dio-boundary-3038081899
  content-disposition: form-data; name=\&quot;code_verifier\&quot;
  
  application_application_application_application_application
  ----dio-boundary-3038081899
  content-disposition: form-data; name=\&quot;client_id\&quot;
  
  courier_mobile_app
  ----dio-boundary-3038081899
  content-disposition: form-data; name=\&quot;client_secret\&quot;
  
  secret
  ----dio-boundary-3038081899--
  &quot; --compressed &quot;https://auth.dev-drive.XXX.dev/connect/token&quot;
  </code></pre>
  <p>I tried this way</p>
  <pre><code>dio.options.headers['content-Type'] = 'application/x-www-form-urlencoded';
  response = await dio.post(urlString4, data: loginBody4);
  </code></pre>
  <p>but result is the same:</p>
  <pre><code>content-type: multipart/form-data; boundary=--dio-boundary-3428087265
  </code></pre>