TypeError (wrong argument type OpenSSL::Cipher (expected Data))

When I implement the example from the readme, on the cipher.encrypt line I get the above error. My implementation:

```
    require 'aead'

    mode   = AEAD::Cipher.new('AES-256-GCM')
    key    = mode.generate_key
    nonce  = mode.generate_nonce

    cipher    = mode.new(key)
    aead      = cipher.encrypt(nonce, 'authentication data', 'plaintext')
```

ruby 2.2.2
