If anyone calls a message encryptor in the console it will
show the secret of the encryptor.

By overriding the `inspect` method to only show the class name we can
avoid accidentally outputting sensitive information.

Before:

```ruby
ActiveSupport::MessageEncryptor.new(secret, cipher: "aes-256-gcm").inspect
"#<ActiveSupport::MessageEncryptor:0x0000000104888038 ... @secret=\"\\xAF\\bFh]LV}q\\nl\\xB2U\\xB3 ... >"
```

After:

```ruby
ActiveSupport::MessageEncryptor.new(secret, cipher: "aes-256-gcm").inspect
"#<ActiveSupport::MessageEncryptor:0x0000000104888038>"
```