Fix wrong type name serialization due to the plus sign '+' is escaped.

[huoshan12345]

What Changed

• Use JavaScriptEncoder.UnsafeRelaxedJsonEscaping for Utf8JsonWriter in DefaultJsonSerializer

Why This Change

• When default encoder is used, the '+' in a type name will be escaped to \u002B in the method SerializeObject, which will lead to an error when trying to GetType by type name in the method DeserializeObject.
• Using JavaScriptEncoder.UnsafeRelaxedJsonEscaping can avoid the escape.
• See related issue on + sign is changing into \u002B using JsonSerializer.Serialize dotnet/runtime#35281

[huoshan12345]

I don't know why the test case Issues497_GetCountAsync_Check_Expires_Test failed, which passed on my PC.
It should not caused by my PR, so rerunning the tests might fix it.