DefaultHttpClient { get; set; } = new HttpClient(); can cause problems

[Meister1977]

Hi,

I see, your code should run with different .net versions, but adding a static default HttpClient is not a good idea in .Net Core, because of this:

https://docs.microsoft.com/en-us/dotnet/architecture/microservices/implement-resilient-applications/use-httpclientfactory-to-implement-resilient-http-requests

I may use the HttpClientFactory and because in .Net core if somebody uses DI, and register the class as Singleton, you should implement
public static GoCardlessClient Create(string accessToken, Environment environment, HttpClientFactory httpClientFactory)
too.

Thanks,
Lőrinc