Fix C# unsigned int generation (#60)
Fix C# unsigned int generation (#60)

This fixes a problem in the Encode and Decode methods when generating unsigned types (e.g. UInt32.cs).

Now the code generated should match the one living in dotnet-stellar-sdk line by line.
stellar/xdrgen,stellar/xdrgen,stellar/xdrgen,stellar/xdrgen,stellar/xdrgen,stellar/xdrgen