Add option to enable dependency loading in production

This will make easier to applications that rely on having const_missing
hooks in production upgrade to Rails 5.

This option is going to be remove in the future and the default behavior
will be to disable the dependency loading.

guides/source/configuring.md

@@ -94,6 +94,8 @@ application. Accepts a valid week day symbol (e.g. `:monday`).
 
 * `config.eager_load_paths` accepts an array of paths from which Rails will eager load on boot if cache classes is enabled. Defaults to every folder in the `app` directory of the application.
 
+* `config.enable_dependency_loading` when true, enable the autoload loading behavior even if the application is eager loaded and have `cache_classes` as true. Default to false.
+
 * `config.encoding` sets up the application-wide encoding. Defaults to UTF-8.
 
 * `config.exceptions_app` sets the exceptions application invoked by the ShowException middleware when an exception happens. Defaults to `ActionDispatch::PublicExceptions.new(Rails.public_path)`.

railties/lib/rails/application/finisher.rb

@@ -176,7 +176,7 @@ def self.complete(_state)
 
       # Disable dependency loading during request cycle
       initializer :disable_dependency_loading do
-        if config.eager_load && config.cache_classes
+        if config.eager_load && config.cache_classes && !config.enable_dependency_loading
           ActiveSupport::Dependencies.unhook!
         end
       end