binding ruby: Fix a typo

RHASH_ENPTY_P ->
RHASH_EMPTY_P

GitHub: ref #42
binding ruby: Add Milter::Client::MailTransactionShelf

GitHub: ref #42
binding ruby: Fix test case for Milter::ClientContext#mail_transaction_shelf

GitHub: ref #42
binding ruby: Add missing require

GitHub: ref #42
binding ruby: Use Milter::Client::MailTransactionShelf

GitHub: ref #42
binding ruby: Serialize as YAML

GitHub: ref #42
binding ruby: Fix test

GitHub: ref #42