Places the CommonResponse struct at the *end* of all IAM responses. (#8537)

* Places the CommonResponse struct at the end of all IAM responses, rather than the start.

* iam: fix error response request id layout

* iam: add XML ordering regression test

* iam: share request id generation

---------

Co-authored-by: Aaron Segal <aaron.segal@rpsolutions.com>
Co-authored-by: Chris Lu <chris.lu@gmail.com>

weed/iamapi/iamapi_management_handlers.go

@@ -997,9 +997,7 @@ func (iama *IamApiServer) DoActions(w http.ResponseWriter, r *http.Request) {
 		changed = false
 	default:
 		errNotImplemented := s3err.GetAPIError(s3err.ErrNotImplemented)
-		errorResponse := ErrorResponse{}
-		errorResponse.Error.Code = &errNotImplemented.Code
-		errorResponse.Error.Message = &errNotImplemented.Description
+		errorResponse := newErrorResponse(errNotImplemented.Code, errNotImplemented.Description)
 		s3err.WriteXMLResponse(w, r, errNotImplemented.HTTPStatusCode, errorResponse)
 		return
 	}