Fix missing options on redis objects in subclasses

I encountered an issue where in subclasses (I'm using STI in a rails project), the Redis Objects that are created don't have the options defined in the parent class.

I'm mostly using lists with maxlength, and in the subclasses the maxlength argument is not present, and the lists keep growing.

I have created a [test case](https://gist.github.com/arnaudl/19a0d157c0a93d99452e) which should be easier to understand :)

I've made a fix which adds a `redis_options` method, following the example of `redis_field_redis` and `redis_field_key` methods.

This should also solve the issue created by my colleague a few weeks ago: #148