Fix hmac bindings

[villesau]

Previously it was not possible to do something like this:

Crypto.Hmac.(create(`SHA256, hmacKey)->appendString(str)->toHex)

which is totally valid.

Hmac node docs: https://nodejs.org/api/crypto.html#crypto_hmac_update_data_inputencoding Documentation for allowed encodings (implemented only hex this time): https://nodejs.org/api/buffer.html#buffer_buffers_and_character_encodings