Ed25519.KeyExchange generates the same shared keys

Fix for https://github.com/CodesInChaos/Chaos.NaCl/issues/4

(Note that I don't use Visual Studio and thus no working `Microsoft.VisualStudio.TestTools.UnitTesting`. Consequently, I haven’t actually compiled the test project with the additional test method. Also, I manually added the new file `unmultiplied_a.cs` in the `.csproj` files. I hope I don’t break anything.)