Duplicate Rails.logger before assigning it to the SDK

If we don't duplicate it, logger configuration made to the SDK's logger
will be applied to the Rails logger as well.

For example, in an Rails app:

```rb
Sentry.init do |config|
  config.logger.level = Logger::WARN
end
```

Will make the Rails logger's level to be `Logger::WARN` as well.

sentry-rails/lib/sentry/rails/configuration.rb

@@ -12,7 +12,7 @@ class Configuration
       @excluded_exceptions = @excluded_exceptions.concat(Sentry::Rails::IGNORE_DEFAULT)
 
       if ::Rails.logger
-        @logger = ::Rails.logger
+        @logger = ::Rails.logger.dup
       else
         @logger.warn(Sentry::LOGGER_PROGNAME) do
           <<~MSG

sentry-rails/spec/dummy/test_rails_app/app.rb

@@ -53,7 +53,7 @@ def self.name
   app.config.action_controller.view_paths = "spec/dummy/test_rails_app"
   app.config.hosts = nil
   app.config.secret_key_base = "test"
-  app.config.logger = Logger.new(nil)
+  app.config.logger = ActiveSupport::Logger.new(nil)
   app.config.eager_load = true
   app.config.active_job.queue_adapter = :test
 

sentry-rails/spec/sentry/rails_spec.rb

@@ -35,8 +35,12 @@
     end
 
     describe "logger detection" do
-      it "sets Sentry.configuration.logger correctly" do
-        expect(Sentry.configuration.logger).to eq(Rails.logger)
+      it "sets a duplicated Rails logger as the SDK's logger" do
+        expect(Sentry.configuration.logger).to be_a(ActiveSupport::Logger)
+        Sentry.configuration.logger.level = ::Logger::WARN
+        # Configuring the SDK's logger should not affect the Rails logger
+        expect(Rails.logger.level).to eq(::Logger::DEBUG)
+        expect(Sentry.configuration.logger.level).to eq(::Logger::WARN)
       end
 
       it "respects the logger set by user" do