[PATCH] to_xml doesn't support nested structures and other inconstancies

I've ran into a few problems using to_xml.

• On an ActiveRecord model, calling to_xml(:methods => :custom_method) where custom_method returns a hash, the hash is simply converted to a string and not represented in XML.
• In the same situation, if custom_method returns an array of values, the array is mashed together as a single string.
• to_xml(:camelize => true) doesn't always play nice with arrays. The inner tags might be camelized, but the outer tag never seems to be properly camelized.

These are just a few problems, but more generally I've repeatedly run into problems where to_json can simply handle converting any object to JSON, but to_xml will balk, convert objects into not so useful strings, or simply not do what's expected.

I began trying to fix these problems, but quickly fell down a rabbit-hole of completely refactoring how to_xml works. So now I have a patch where to_xml is organized and works much like to_json does now. It fixes all of these problems with certain objects not supporting to_xml by allowing any object to define how it will be coerced into an XML value and how it will build its XML tags if necessary.

Here's a quick example showing the previous problems and how things work with the patch, which I think would be the expected behavior:

class Report < ActiveRecord::Base
  def item_ids
    [8, 9]
  end

  def metadata
    { :color => "blue", :options => [:blue, :red] }
  end
end

report.to_xml(:methods => [:item_ids, :metadata])

Previously, this would generate:

<report>
  ...
  <item-ids>89</item-ids>
  <metadata>colorblueoptionsbluered</metadata>
</report>

Now it generates:

<report>
  ...
  <item-ids type="array">
    <item-id type="integer">8</item-id>
    <item-id type="integer">9</item-id>
  </item-ids>
  <metadata>
    <color>blue</color>
    <options type="array">
      <option type="symbol">blue</option>
      <option type="symbol">red</option>
    </options>
  </metadata>
</report>

I've introduced new tests to test these new cases. The patch maintains backwards compatibility with all previous tests. The one exception is a newer ActiveModel test where a method returns a hash and it was expecting to be converted to YAML. ActiveRecord serialized_attributes are still converted to YAML, but since ActiveModel doesn't know about serialized_attributes, the hash returned there is converted into nested XML.

Any feedback or questions are welcome. Hopefully this patch makes to_xml behave more as expected with other data types, and also makes it more flexible to customize how objects respond to XML encoding. The patch provided is against the latest master, but if this patch seems desirable, is there any chance of getting these changes implemented on 2.3 stable? I started out patching 2-3-stable, so I have most of this working there, but since this patch grew a bit more than I expected, I thought I'd get feedback first. But I can provide a 2.3 patch if it's wanted.

Here are a couple of older bug reports documenting some of these same issues: #458 and #1576