[Fixes #1528] Default reference assemblies made extensible in DefaultAssemblyProvider

@pranavkm @yishaigalatzer 
This assumes the base implementation contains m.aspnet.mvc.core (and we might just remove/rename that assembly at some point), so it's a good idea to include the reverse test as well where you verify the default implementation contains it, but the overriden one doesn't and provides what you are looking for.