Fix issue while adding XML for requests diff

We had an issue that was adding the <?xml version="1.0"??> after the
first node (<request...>), now is fixed and is the first node of the XML

Co-authored-by: Christian Bruckmayer <cbruckmayer@suse.com>
Co-authored-by: Dany Marcoux <dmarcoux@suse.com>
Co-authored-by: David Kang <dkang@suse.com>
Co-authored-by: Eduardo Navarro <enavarro@suse.com>
Co-authored-by: Saray Cabrera Padrón <scabrerapadron@suse.de>
Co-authored-by: Victor Pereira <vpereira@suse.com>

src/api/app/controllers/request_controller.rb

@@ -185,7 +185,7 @@ def request_command_diff
         # Inject backend-provided XML diff into action XML:
         builder = Nokogiri::XML::Builder.new
         action.render_xml(builder)
-        xml_request.add_child(builder.to_xml)
+        xml_request.add_child(builder.doc.root.to_xml)
         xml_request.at_css('action').add_child(action_diff)
       else
         diff_text += action_diff