Fixed a bug when the JsonSerializerSettings are changed

In my configuration method I change a ``JsonSerializerSettings``
```
  JsonConvert.DefaultSettings = () =>
                new JsonSerializerSettings
                {
                    ContractResolver = new CamelCasePropertyNamesContractResolver(),
                    NullValueHandling = NullValueHandling.Ignore,
                    MissingMemberHandling = MissingMemberHandling.Ignore,
                    DefaultValueHandling = DefaultValueHandling.IgnoreAndPopulate,
                    TypeNameHandling = TypeNameHandling.None,
                };
```
And with such settings I can register as many users with the same user name as I want.

This PR fixes the issue.