XDR specification does not enforce string encoding but UTF-8 used

XDR specification does not enforce string encoding but strings are encoded/decoded as UTF-8. I think it's fine for presentation but we should maybe add a warning in README. Currently if you encode and decode JSON/XDR with a string with non-utf8 chars the final JSON/XDR will be different than original one.

Similar issue: https://github.com/stellar/js-xdr/pull/22