Problem in call Microsoft RIST API by UnityWebRequest

I want to call microsoft RIST API ( text to speech) in Unity.
When I use WWW, it works fine, I can get the speech WAV file immediately. But when i change to UnityWebRequest in same properties, it always no response until timeout. (The token has generated successfully.)
I don't know why. Could any guys can tell me where is the mistake I make?
Unity version: 2017.3.1

it is my code by WWW:

Code (CSharp):

1.         string text = "<speak version='1.0' xml:lang='en-US'><voice xml:lang='en-US' xml:gender='Female' name='Microsoft Server Speech Text to Speech Voice (en-US, ZiraRUS)'>Today is a good day to die!</voice></speak>";
2.         byte[] data = System.Text.Encoding.UTF8.GetBytes(text);
3.  
4.         Dictionary<string, string> headers = new Dictionary<string, string>();
5.         headers.Add("X-Microsoft-OutputFormat", "riff-8khz-8bit-mono-mulaw");
6.         headers.Add("Content-Type", "application/ssml+xml");
7.         headers.Add("Authorization", "Bearer " + token);
8.  
9.         WWW www = new WWW(url, data, headers);
10.  
11.         yield return www;
12.  
13.         if (www.error != null)
14.         {
15.             Debug.LogError(www.error);
16.         }
17.         else
18.         {
19.             Debug.Log(www.bytes.Length);
20.             File.WriteAllBytes(Application.streamingAssetsPath + "/receive.wav", www.bytes);
21.         }

and, it is my code by UnityWebRequest:

Code (CSharp):

1.         string text = "<speak version='1.0' xml:lang='en-US'><voice xml:lang='en-US' xml:gender='Female' name='Microsoft Server Speech Text to Speech Voice (en-US, ZiraRUS)'>Today is a good day to die!</voice></speak>";
2.         byte[] data = System.Text.Encoding.UTF8.GetBytes(text);
3.  
4.         UnityWebRequest req = new UnityWebRequest(url);
5.         req.method = UnityWebRequest.kHttpVerbPOST;
6.         req.SetRequestHeader("X-Microsoft-OutputFormat", "riff-8khz-8bit-mono-mulaw");
7.         req.SetRequestHeader("Content-Type", "application/ssml+xml");
8.         req.SetRequestHeader("Authorization", "Bearer " + token);
9.  
10.         req.uploadHandler = new UploadHandlerRaw(data);
11.         req.downloadHandler = new DownloadHandlerBuffer();
12.  
13.         req.timeout = 10;
14.  
15.         yield return req.SendWebRequest();
16.  
17.         if (req.error != null)
18.         {
19.             Debug.LogError(req.error + " [code=" + req.responseCode + "]");
20.         }
21.         else
22.         {
23.             if (req.responseCode == 200)
24.             {
25.                 Debug.Log("Success!");
26.                 File.WriteAllBytes(Application.streamingAssetsPath + "/receive.wav", req.downloadHandler.data);
27.             }
28.             else
29.             {
30.                 Debug.LogWarning("Parse Failed! code=" + req.responseCode);
31.             }
32.         }

 

Unity version is 2017.3.1f1
if I use WWW, it will almost return result immediate, so 10 second is long enough I think.
and, I have tried 30 second timeout, the result is same...

 

For anyone still having problems at 2019.3.4f1 and gettting HTTP error 400 - the bodydata in the UnityWebRequest.Post is urlencoded so you can't use it for ssml. See this for a solution https://forum.unity.com/threads/how-to-post-json-in-body.390672/