Do not encrypt otp_secret if attr_encrypted already set

This PR allows users to add encryption themselves using the options they would like. 

In our case, we are using [reidmorrison/symmetric-encryption](https://github.com/reidmorrison/symmetric-encryption) which implements attr_encrypted but does not support the `:key` or `:mode` options. This change allows us to implement the encrypting ourselves without getting a warning for the unsupported options.