x509: Handle non-UTF8 ASN1 strings correctly by converting them to UTF8.

Certificates containing `T61STRING` fields are not decoded properly. For example, `:subject().stateOrProvinceName` shows up as: "Baden-W�rttemberg" instead of "Baden-Württemberg" on this certificate: https://gist.github.com/xnyhps/6678389.

This pull request fixes that by always converting the string to UTF8 before pushing it on the Lua stack.