This commit moves support for the :include serialization option for
serializing associated objects out of ActiveRecord in into ActiveModel.
The following methods support the :include option:
Instances must respond to methods named by the values of the :includes
array (or keys of the :includes hash). If an association method returns
an object that is_a?(Enumerable) (which AR has_many associations do), it
is assumed to be a collection association, and its elements must respond
to :serializable_hash. Otherwise it must respond to :serializable_hash
While here, fix #858, XmlSerializer should not singularize already
singular association names.