Serializing JSON with DataContractJsonSerializer in C# gives trailing null characters (\0)

Question

When serializing a JSON I get trailing null characters which makes my unit test fail. The serializer looks like this:

public static string SerializeJSON<T>(T ObjectToSerialize)
    {
        DataContractJsonSerializer serializer = new DataContractJsonSerializer(ObjectToSerialize.GetType());

        using (MemoryStream memoryStream = new MemoryStream())
        {
            using (var jsonWriter = JsonReaderWriterFactory.CreateJsonWriter(memoryStream, Encoding.UTF8, false, true))
            {
                serializer.WriteObject(jsonWriter, ObjectToSerialize);
            }
            return Encoding.UTF8.GetString(memoryStream.GetBuffer());
        }
    }

I have written a unit test for it that looks like this:

[TestMethod]
    public void SerializeJSON()
    {
        string testJSON = File.ReadAllText(@"Util\\TestCurrency.json");
        Currency currency = StaticExamples.ccy1;
        string json = SerializeUtil.SerializeJSON(currency);
        Assert.AreEqual(testJSON, json);
    }

Which compares the serialized currency to the file example, they should be identical. But the test fails saying:

Message: Assert.AreEqual failed. Expected:<{
"code": "USD",
"currencyId": 1,
"name": "Dollar",
"status": "Active"
}>. 
Actual:<{
"code": "USD",
"currencyId": 1,
"name": "Dollar",
"status": "Active"
}\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0>. 

The trailing characters are null characters from what I understand. Any idea of why they appear? It works to Deserialize this JSON again but my test still fails. This is tha class being serialized:

public class Currency
{       
    public int currencyId { get; set; }
    public string code { get; set; }
    public string name { get; set; }
    public string status { get; set; }
}

EDIT with answer Thanks to @DavidG I changed the way I converted the stream to a string and it works know. This is the generic serializer code for anyone who wants it:

public static string SerializeJSON<T>(T ObjectToSerialize)
    {
        DataContractJsonSerializer serializer = new DataContractJsonSerializer(ObjectToSerialize.GetType());

        using (MemoryStream memoryStream = new MemoryStream())
        {
            using (var jsonWriter = JsonReaderWriterFactory.CreateJsonWriter(memoryStream, Encoding.UTF8, false, true))
            {
                serializer.WriteObject(jsonWriter, ObjectToSerialize);
            }
            memoryStream.Position = 0;
            return new StreamReader(memoryStream).ReadToEnd();
        }
    }