[Fix rubocop#832] Mark some Rails cops as unsafe autocorrection

Fixes rubocop#832.

This commit marks `Rails/ActionControllerFlashBeforeRender`, `Rails/ActionControllerTestCase`,
and `Rails/RootPathnameMethods` cops as unsafe autocorrection to resolve unexpected behaviors.

## Before

Behave as a safe autocorrection (`-a`).

```consle
% echo "File.open(Rails.root.join('db', 'schema.rb'))" |
bundle exec rubocop --stdin example.rb -a --enable-pending-cops --require=rubocop-rails --only Rails/RootPathnameMethods
Inspecting 1 file
C

Offenses:

example.rb:1:1: C: [Corrected] Rails/RootPathnameMethods: Rails.root is a Pathname so you can just append #open.
File.open(Rails.root.join('db', 'schema.rb'))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

1 file inspected, 1 offense detected, 1 offense corrected
====================
Rails.root.join('db', 'schema.rb').open
```

## After

Behave as an unsafe autocorrection (`-A`).

```console
% echo "File.open(Rails.root.join('db', 'schema.rb'))" |
bundle exec rubocop --stdin example.rb -a --enable-pending-cops --require=rubocop-rails --only Rails/RootPathnameMethods
Inspecting 1 file
C

Offenses:

example.rb:1:1: C: [Correctable] Rails/RootPathnameMethods: Rails.root is a Pathname so you can just append #open.
File.open(Rails.root.join('db', 'schema.rb'))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

1 file inspected, 1 offense detected, 1 more offense can be corrected with `rubocop -A`
====================
File.open(Rails.root.join('db', 'schema.rb'))
```

Author: koic

config/default.yml

@@ -73,15 +73,15 @@ Rails/ActionControllerFlashBeforeRender:
   StyleGuide: 'https://rails.rubystyle.guide/#flash-before-render'
   Reference: 'https://api.rubyonrails.org/classes/ActionController/FlashBeforeRender.html'
   Enabled: 'pending'
-  SafeAutocorrect: false
+  SafeAutoCorrect: false
   VersionAdded: '2.16'
 
 Rails/ActionControllerTestCase:
   Description: 'Use `ActionDispatch::IntegrationTest` instead of `ActionController::TestCase`.'
   StyleGuide: 'https://rails.rubystyle.guide/#integration-testing'
   Reference: 'https://api.rubyonrails.org/classes/ActionController/TestCase.html'
   Enabled: 'pending'
-  SafeAutocorrect: false
+  SafeAutoCorrect: false
   VersionAdded: '2.14'
   Include:
     - '**/test/**/*.rb'
@@ -873,7 +873,7 @@ Rails/RootJoinChain:
 Rails/RootPathnameMethods:
   Description: 'Use `Rails.root` IO methods instead of passing it to `File`.'
   Enabled: pending
-  SafeAutocorrect: false
+  SafeAutoCorrect: false
   VersionAdded: '2.16'
 
 Rails/RootPublicPath: