In Unity SDK,Http return xml format string which cause can not covert json error

[sasalicat]

` if (req.isNetworkError || req.isHttpError)
#endif
{
var errorMessage = req.error;

                //
                // Parse JSON from response
                //
                if (!string.IsNullOrEmpty(req.downloadHandler.text))
				{
                    var data = Json.Deserialize<ErrorResponse>(req.downloadHandler.text);
                    if (!string.IsNullOrEmpty(data.error))
					{
                        errorMessage = data.error;
                        throw new HttpException((int)req.responseCode, errorMessage);
                    }
				}

                throw new HttpException((int)req.responseCode, errorMessage);
            }

            return req.downloadHandler.text;`

Here I show a part of function "public async Task Request(string uriMethod, string uriPath, Dictionary<string, object> jsonBody = null, Dictionary<string, string> headers = null)",which in class HTTP.In this part we can see that if error happen in requset, Http cover the req.downloadHandler.text into ErrorResponse by json,other case,It return req.downloadHandler.text directly.But I notice that req.downloadHandler.text is actually a XML format string,which obviously cannot covert by json,As a result,every time when http request fail a Unity error was triggered.
I try add Content-Type to header to order server return json format,but it doesn`t work.
Am I missing some setting for that?
in Unity SDK version:0.16.13