Convenience method for setting bit_fields using symbols on column_name

I've been messing around with migrating some models from DataMapper over to Sequel.  Sequel-bit_fields has been super helpful.  

My suggested change more or less adds a similar behavior from the Flag type found in dm-types.  This allows assigning bit_fields by passing symbols into the model's associated column_name/property.  Let me know what you think.
```
self.send("#{bit_field[:name]}=", args.include?(bit_field[:name]))
```