Methods that return nil should not be considered YAML

This is a direct port of @jaw6's pull request
https://github.com/rails/rails/pull/492. His cleanly applied to Rails
v3.1 and v3.2, and this cleanly applies to v3.0.

With yesterday's security patches
http://weblog.rubyonrails.org/2013/1/8/Rails-3-2-11-3-1-10-3-0-19-and-2-3-15-have-been-released/
there is now an issue with Rails v3.0 serving XML to any of the latest
versions of ActiveResource.

Without this, Rails v3.0 can serve XML to ActiveResource consumers that
will see `Hash::DisallowedType: Disallowed type attribute: "yaml"`
Merge pull request #8853 from zmoazeni/3-0-xml-serialization-fix

Methods that return nil should not be considered YAML