Set ActiveResource::Base.logger during boot

When left un-configured during boot, default the value of
`ActiveResource::Base.logger` to `Rails.logger`.

This change draws inspiration from the corresponding lines in the
`active_record/railtie.rb` file's [active_record.logger][] initializer.

[active_resource.logger]: https://github.com/rails/rails/blob/1b327ad4ed0fe5e8c2f1f795e2cf63f90a9833b5/activerecord/lib/active_record/railtie.rb#L97-L99

Author: seanpdoyle

lib/active_resource/railtie.rb

@@ -27,5 +27,9 @@ class Railtie < Rails::Railtie
         app.deprecators[:active_resource] = ActiveResource.deprecator
       end
     end
+
+    initializer "active_resource.logger" do
+      ActiveSupport.on_load(:active_resource) { self.logger ||= ::Rails.logger }
+    end
   end
 end