RESTful web service, but leaving inbound XML intact

I'm developing a Rails application which has to process orders via a web
service. I want to develop this as a RESTful one, and have worked out that
since this is an "INSERT" the call should be a POST.

However first on my list of business requirements is that the inbound XML has
to be stored "untouched".

It seems that Rails helpfully parses the XML inbound for me.

If someone could describe to me how to add a RESTful web service to Rails,
which calls a method orders/public_order and which gives me the raw XML to
work with that would be really helpful.