Add support for nested wrappers in the :in clause

[jpshackelford]

While the following worked for reads, it would create xml elements which contain a '/' on writes which crashes nokogiri jruby1.5.0 and is produced but invalid in other versions/platforms. This fix allows ROXML to properly reconstruct xml with a nested wrapper.

class LibraryWithFines
  include ROXML
  xml_name 'library'
  xml_accessor :name
  xml_accessor :fines,
               :as   => { :key => 'name', :value => 'desc' },
               :from => 'fine',
               :in   => 'policy/fines'
end