Support for BCL immutable collections

git-svn-id: http://protobuf-net.googlecode.com/svn/trunk@665 b72047a4-3652-0410-9617-b3994939e97b
Fixes for BCL immutable

git-svn-id: http://protobuf-net.googlecode.com/svn/trunk@666 b72047a4-3652-0410-9617-b3994939e97b