We should make it as natural for Ruby developers as possible.

I don't know how method missing works but, can we make it work the same way we do for fields of other resources? If this is not possible, we should at least make it work like a Hash (and implement or override the methods that make sense).

In the Java SDK we use "put" and "get" because CustomData implements the Map interface, which requires those methods and it's also natural for Java developers to use those methods when interacting with a map.