Error in a Mutation Execution of 6.0.1 Nuget Version

Hi

```csharp
internal class WebApiAuthTokenService
{

private static GraphQLHttpClient GraphQLClientForCreateToken;

private readonly string _webApiAddress;

public WebApiAuthTokenService(string WebApiAddress)
{
_webApiAddress = WebApiAddress;

GraphQLClientForCreateToken = new GraphQLHttpClient(_webApiAddress, new NewtonsoftJsonSerializer());
}

public async Task createToken(string login, string Password)
{

var heroRequest = new GraphQLRequest
{
Query = @"mutation createToken {
createToken(login: """ + login + @""", password: """ + Password + @""")
{
token
}
}"
};

var ApiResponse = await GraphQLClientForCreateToken.SendMutationAsync(heroRequest);

return ApiResponse.Data;

}

}
```

Just by upgrading from version 6.0.0 to version 6.0.1 of the package, the above code starts reporting the exception with the text "The HTTP request failed with status code OK".