Check with defined? to automatically load extensions

[r7kamura]

What about checking with defined?(Rack) and defined?(Rails::Railtie) to automatically load the available extensions? This way, the user will not have to require: ... .

This method is often used in Rails plugins. For example, here is an example of kaminari, a well-known plugin for pagination:

• https://github.com/kaminari/kaminari/blob/ff07a285215cfb8a5f70515e86decf4a16ff643c/kaminari-core/lib/kaminari/core.rb#L21-L24