Fixed immutable list enumerator.
Improved API compatibility with the MS nuget package.
Merge remote-tracking branch 'mkrueger/master'

Conflicts:
	System.Collections.Immutable/System.Collections.Immutable/ImmutableList.cs
	System.Collections.Immutable/System.Collections.Immutable/IndexedAvlNode.cs
	UnitTests/ImmutableListTests.cs