Deep dup default confing before letting specs modify it

All cops config:
```ruby
RuboCop::ConfigLoader.default_configuration.for_all_cops.dig('RSpec', 'Language')
```

specifically `for_all_cops` is memoized in `Config`:
```ruby
# rubocop-0.89.1/lib/rubocop/config.rb:132:

def for_all_cops
  @for_all_cops ||= self['AllCops'] || {}
end
```

It's not so obvious to notice, since in this cop the example that is
modifying the language is in its own context, and it runs last. Even if
you add an example after it, due to how RSpec orders examples, it will
still be executed last. However, if you add a `context` with an
example, you'll see that the language changes leaked.

Regular `freeze` and `dup` won't work:

    { 'Language' =>    # <= this is dupped
      { 'Includes' =>  # <= this is the same Hash in the original and dup
        { 'Example' =>
          ['it_behaves_like, 'include_examples', ..., 'it_has_special_behavior']

So, we need to deep dup the default config to avoid config modification
leakages.

spec/rubocop/cop/rspec/empty_example_group_spec.rb

@@ -212,20 +212,10 @@
   end
 
   context 'when a custom include method is specified' do
-    let(:language) do
-      RuboCop::ConfigLoader
-        .default_configuration
-        .for_all_cops
-        .dig('RSpec', 'Language')
-    end
-    let(:all_cops_config) do
-      cfg = { 'TargetRubyVersion' => ruby_version }
-      cfg['TargetRailsVersion'] = rails_version if rails_version
-      cfg.merge({ 'RSpec' => { 'Language' => language } })
-    end
-
     before do
-      language['Includes']['Example'] << 'it_has_special_behavior'
+      all_cops_config
+        .dig('RSpec', 'Language', 'Includes', 'Example')
+        .push('it_has_special_behavior')
     end
 
     it 'ignores an empty example group with a custom include' do

spec/shared/default_rspec_language_config_context.rb

@@ -4,12 +4,20 @@
   let(:all_cops_config) do
     cfg = { 'TargetRubyVersion' => ruby_version }
     cfg['TargetRailsVersion'] = rails_version if rails_version
-    cfg.merge({
-                'RSpec' => {
-                  'Language' => RuboCop::ConfigLoader
-                                  .default_configuration
-                                  .for_all_cops.dig('RSpec', 'Language')
-                }
-              })
+    default_language = RuboCop::ConfigLoader
+      .default_configuration
+      .for_all_cops.dig('RSpec', 'Language')
+    cfg.merge('RSpec' => { 'Language' => deep_dup(default_language) })
+  end
+
+  def deep_dup(object)
+    case object
+    when Array
+      object.map { |item| deep_dup(item) }
+    when Hash
+      object.transform_values(&method(:deep_dup))
+    else
+      object # only collections undergo modifications and need duping
+    end
   end
 end