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/iam/responses_test.go

@@ -0,0 +1,26 @@
+package iam
+
+import (
+	"encoding/xml"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestListUsersResponseXMLOrdering(t *testing.T) {
+	resp := ListUsersResponse{}
+	resp.SetRequestId()
+
+	output, err := xml.Marshal(resp)
+	require.NoError(t, err)
+
+	xmlString := string(output)
+	listUsersResultIndex := strings.Index(xmlString, "<ListUsersResult>")
+	responseMetadataIndex := strings.Index(xmlString, "<ResponseMetadata>")
+
+	assert.NotEqual(t, -1, listUsersResultIndex, "ListUsersResult should be present")
+	assert.NotEqual(t, -1, responseMetadataIndex, "ResponseMetadata should be present")
+	assert.Less(t, listUsersResultIndex, responseMetadataIndex, "ListUsersResult should appear before ResponseMetadata")
+}