Minimumlevel partial fix with IConfigurationRoot.Providers

[HexHunter97]

A partial fix for minimumlevel not being correctly overriden by higher priority providers.

The fix only works for the .NET 4.6.1+ and .NET Standard builds as the IConfigurationRoot interface did not have Providers in .NET 4.5.1.

I understand this is more of an annoyance than a real issue, but it's easy to miss.

[nblumhardt]

Thanks! 👍 As it's a slightly tricky scenario, it would be great to have a test for this behavior, so we can lessen the risk of regression in the future.

[HexHunter97]

I realise the long lines are less than desirable, but I don't know how to break them up properly wihout without taking up too much vertical space. :( Any advice on the matter welcome.

[nblumhardt]

Sorry about the long delay on this; merging to the 3.5.0-dev-* version now.

[HexHunter97]

It's ok, there are other priorities sometimes. Thanks for letting it in!