Serialized methods that return nil should not be considered YAML

[jaw6]

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:

<person>
  <name>John Doe</name>
  <preferences type="yaml" nil="true"></preferences>
</person>

However, in many cases, the 'type="yaml"' part is unnecessary, and potentially misleading.

With this patch, the equivalent to the above XML would be:

<person>
  <name>John Doe</name>
  <preferences nil="true"></preferences>
</person>

[josevalim]

Could you please rebase this pull request? Thanks a lot!