Fixed memory leak of IHub instances. This resolves #3208.  DefaultDepend...

...encyResolver will now avoid storing IHub instances in _trackedDisposables.

- Unit test included.  Without the fix, the test will fail.
- I could not locate an existing file/class for DefaultDependencyResolver tests, so I created a new one. Please advise if not correct.
- This fix needs to also go into 2.1.x and 1.2.x.  I can create multiple PRs for that if needed. Please advise.
You shouldn't dereference the resolver prior to using the WeakReference to assert that the resolver doesn't reference the IUntrackedDisposable.

You also shouldn't dispose the resolver before checking the WeakReference, since the DefaultDependencyResolver clears the _trackedDisposables HashSet in Dispose.

Of course, it won't be as simple to assert that the IUntrackedDisposable hasn't been disposed after you successfully verified that there are no strong references to it. However, I think it's safe to assume that if the resolver isn't holding a reference to it, it can't be disposing it either. If you really wan't to verify that it's not disposed, you can do that in another test