Fix bug to ensure correct provider disposal

[mjcheetham]

We are not correctly disposing of the host providers registered with the host provider registry. Fix this by actually calling Dispose() on the correct objects(!)

[derrickstolee]

I see... foreach loops don't work correctly on ValueCollections, so you need to convert it to an enumerator?

The documentation example says this loop should work.

[mjcheetham]

@derrickstolee ..

I see... foreach loops don't work correctly on ValueCollections, so you need to convert it to an enumerator?

The documentation example says this loop should work.

The problem is that _hostProviders is of type IDictionary<HostProviderPriority, ICollection<IHostProvider>>, which means _hostProviders.Values is actually of type ICollection<ICollection<IHostProvider>>.

I need to unpack those outer collections.

[derrickstolee]

...of course. .SelectMany() is different than .Select(). I haven't had to think about LINQ for a long time.