FIX: Broken group messages inboxes when group name is mixed case (#22183)

This is a follow up to 1cbc65ba79 where
visiting a group with a mixed case name would result in an error.

spec/system/page_objects/pages/user_private_messages.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+module PageObjects
+  module Pages
+    class UserPrivateMessages < PageObjects::Pages::Base
+      def visit(user)
+        page.visit "/u/#{user.username}/messages"
+        self
+      end
+
+      def visit_group_inbox(user, group)
+        page.visit "/u/#{user.username}/messages/group/#{group.name}"
+        self
+      end
+
+      def has_right_inbox_dropdown_value?(value)
+        has_css?(".user-nav-messages-dropdown .combo-box-header[data-name='#{value}']")
+      end
+    end
+  end
+end

spec/system/viewing_user_private_messages_spec.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+describe "Viewing user private messages", type: :system do
+  fab!(:user) { Fabricate(:user, username: "mIxed_caSE_usERNAME") }
+  fab!(:user2) { Fabricate(:user) }
+
+  let(:user_private_messages_page) { PageObjects::Pages::UserPrivateMessages.new }
+
+  before { sign_in(user) }
+
+  describe "when the user has group messages" do
+    fab!(:group) do
+      Fabricate(:group, name: "miXeD_caSE_name", has_messages: true).tap { |g| g.add(user) }
+    end
+
+    before { SiteSetting.personal_message_enabled_groups = Group::AUTO_GROUPS[:everyone] }
+
+    it "allows the user to view the default messages inbox" do
+      user_private_messages_page.visit(user)
+
+      expect(user_private_messages_page).to have_right_inbox_dropdown_value("Inbox")
+    end
+
+    it "allows the user to view the group messages inbox of a group" do
+      user_private_messages_page.visit_group_inbox(user, group)
+
+      expect(user_private_messages_page).to have_right_inbox_dropdown_value("miXeD_caSE_name")
+    end
+  end
+end