(Rebased version of #200 (commits))
The way ActiveModel computes the type of serializable methods and attributes, any that return nil will be described as being YAML.
This produces XML like:
<preferences type="yaml" nil="true"></preferences>
However, in many cases, the 'type="yaml"' part is unnecessary, and potentially misleading.
With this patch, the equivalent to the above XML would be:
Methods that return nil should not be considered YAML