jooby: Cookie`secret.getBytes()` and `value.getBytes()` use the platform default charset

[xsalefter]

secret.getBytes() and value.getBytes() use the platform default charset, which can produce different signatures across environments and break cookie validation (and is generally unsafe for cryptographic operations). Use an explicit charset (e.g., UTF-8) for both the secret and value bytes.

Originally posted by @Copilot in #194 (comment)

[xsalefter]

Fixed by this PR : #198