fix(sts): Fix the problem that the HttpResponse returned by getting STS is null

[zhenlei520]

AlibabaCloudCredentialsProvider provider = new AccessKeyCredentialProvider("<your-access-key-id>", "<your-access-key-secret>");
IClientProfile profile = DefaultProfile.GetProfile("cn-hangzhou");
DefaultAcsClient client = new DefaultAcsClient(profile, provider);

var request = new AssumeRoleRequest
{
    RoleArn = "<your-role-arn>",
    RoleSessionName = "<your-role-session-name>",
    DurationSeconds = 3600
};
try
{
    var response = client.GetAcsResponse(request);
    Assert.IsNotNull(response.HttpResponse);// Tip: The value of HttpResponse obtained here is null
    Console.WriteLine(System.Text.Encoding.Default.GetString(response.HttpResponse.Content));
}
catch (ServerException e)
{
    Console.WriteLine(e);
}
catch (ClientException e)
{
    Console.WriteLine(e);
}

[CLAassistant]

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