[rubygems/rubygems] Fix incorrect gem uninstall --all message

Currently `gem uninstall --all` shows the following incorrect message
after completion:

```
INFO:  Uninstalled all gems in
```

Now it shows something that actually makes sense like:

```
INFO:  Uninstalled all gems in /home/deivid/Code/rubygems/tmp/test_rubygems_6986/gemhome
```

https://github.com/rubygems/rubygems/commit/8fa05e8f8c

lib/rubygems/commands/uninstall_command.rb

@@ -143,7 +143,7 @@ that is a dependency of an existing gem.  You can use the
       uninstall_gem spec.name
     end
 
-    alert "Uninstalled all gems in #{options[:install_dir]}"
+    alert "Uninstalled all gems in #{options[:install_dir] || Gem.dir}"
   end
 
   def uninstall_specific

test/rubygems/test_gem_commands_uninstall_command.rb

@@ -361,6 +361,7 @@ class TestGemCommandsUninstallCommand < Gem::InstallerTestCase
     end
 
     assert_equal %w[default-1], Gem::Specification.all_names.sort
+    assert_equal "INFO:  Uninstalled all gems in #{@gemhome}", @ui.output.split("\n").last
   end
 
   def test_execute_outside_gem_home