Update Client.cs

Isn't it better to receive InvalidCast than return null and then get weird NullReference somewhere else?
Merge branch 'patch-2' of git://github.com/abatishchev/IronScheme into abatishchev-patch-2

git-svn-id: https://ironscheme.svn.codeplex.com/svn@101729 2327b42d-5241-43d6-9e2a-de5ac946f064