[rubygems/rubygems] Fix gem update --system for already installed version of rubygems-update

https://github.com/rubygems/rubygems/commit/c167d513a7

lib/rubygems/commands/update_command.rb

@@ -287,7 +287,7 @@ command to remove old versions.
     check_oldest_rubygems version
 
     installed_gems = Gem::Specification.find_all_by_name 'rubygems-update', requirement
-    installed_gems = update_gem('rubygems-update', version) if installed_gems.empty?
+    installed_gems = update_gem('rubygems-update', version) if installed_gems.empty? || installed_gems.first.version != version
     return if installed_gems.empty?
 
     version = installed_gems.first.version

test/rubygems/test_gem_commands_update_command.rb

@@ -157,6 +157,40 @@ class TestGemCommandsUpdateCommand < Gem::TestCase
     assert_empty out
   end
 
+  def test_execute_system_update_installed
+    spec_fetcher do |fetcher|
+      fetcher.download 'rubygems-update', 8 do |s|
+        s.files = %w[setup.rb]
+      end
+    end
+
+    @cmd.options[:args]          = []
+    @cmd.options[:system]        = true
+
+    @cmd.execute
+
+    spec_fetcher do |fetcher|
+      fetcher.download 'rubygems-update', 9 do |s|
+        s.files = %w[setup.rb]
+      end
+    end
+
+    @cmd = Gem::Commands::UpdateCommand.new
+    @cmd.options[:args]          = []
+    @cmd.options[:system]        = true
+
+    use_ui @ui do
+      @cmd.execute
+    end
+
+    out = @ui.output.split "\n"
+    assert_equal "Updating rubygems-update", out.shift
+    assert_equal "Installing RubyGems 9", out.shift
+    assert_equal "RubyGems system software updated", out.shift
+
+    assert_empty out
+  end
+
   def test_execute_system_specific
     spec_fetcher do |fetcher|
       fetcher.download 'rubygems-update', 8 do |s|