[Authentication]405 Method not allowed

[saber-wang]

Due to the change of #13 , the authentication of OAuthClient is interrupted and the authentication mode of

Docker.Registry.DotNet/src/Docker.Registry.DotNet/OAuth/OAuthClient.cs

Lines 26 to 47 in 63c268c

	var queryString = new QueryString();
	queryString.AddIfNotEmpty("service", service);
	queryString.AddIfNotEmpty("scope", scope);
	var builder = new UriBuilder(new Uri(realm))
	{
	Query = queryString.GetQueryString()
	};
	var request = new HttpRequestMessage(HttpMethod.Get, builder.Uri);
	if (username != null && password != null)
	{
	// https://gist.github.com/jlhawn/8f218e7c0b14c941c41f
	var bytes = Encoding.UTF8.GetBytes($"{username}:{password}");
	var parameter = Convert.ToBase64String(bytes);
	request.Headers.Authorization = new AuthenticationHeaderValue("Basic", parameter);
	}

is missing

[Jaben]

A PR would be ideal as I don't have a lot of ways to test this system.