Allow xml: name: root to skip a nested XML element.

[gabesmed]

This PR sets a special

xml:
  name: root

in a swagger descriptor for the pru On encountering this flag, the first property of the object is converted directly into XML, rather than being nested.

So:

xml:
  name: root
properties:
  data:
    example: 123

will be converted to <data>123</data>, not <root><data>123</data></root>.