Using config with fallback as fallback for another config looses fallback values

[IgorFedchenko]

The following fails:

var config = ConfigurationFactory.ParseString("a = 1");
var fallback = ConfigurationFactory.ParseString("b = 2");
var configWithFallback = config.WithFallback(fallback);

var anotherConfig = ConfigurationFactory.ParseString("c = 1");
var merged = anotherConfig.WithFallback(configWithFallback);
// Here we loose value that was fallback for configWithFallback
merged.GetInt("b").Should().Be(2);

When getting values from fallback config, need to get their fallbacks as well. Most likely this can be resolved with simple using Fallback.Root instead of Fallback.Value when building final config.

[IgorFedchenko]

Ooops... Sorry, @Aaronontheweb - just verified, this issue is a mistake. I had a typo in the test, and fixed it in issue description only after fixed the Config implementation, so thought that it started working because of my fix...

Closing this.