changing get_db_prep_save to get_db_value

I'm trying to make this field work with an EncryptedTextField I have, and realized that get_db_prep_save isn't the best place for the encoding to happen.

get_db_prep_save should be used when there's a custom database type like connection.Database.Binary or something. See here: https://docs.djangoproject.com/en/dev/howto/custom-model-fields/#converting-query-values-to-database-values

get_prep_value should be a better place to do this, and is working great for my purposes at least.

Figured I'd send a pull request in case you agreed! This will affect backwards compatibility for some.