Update the install generator to set new defaults for new apps

We want to change these defaults for all apps eventually, so making sure
new apps are generated with them by default will make this an easier
transation to some in the future.

README.md

@@ -261,7 +261,7 @@ class ApplicationResponder < ActionController::Responder
 end
 ```
 
-_Note: these defaults may change in a future major release of responders._
+_Note_: the application responder generated for new apps already configures a different set of defaults: `422 Unprocessable Entity` for errors, and `303 See Other` for redirects. _Responders may change the defaults to match these in a future major release._
 
 ## Examples
 

lib/action_controller/respond_with.rb

@@ -97,6 +97,8 @@ def clear_respond_to
  # renders a default action, which is <tt>:new</tt> for a
  # +post+ request or <tt>:edit</tt> for +patch+ or +put+,
  # and the status is set based on the configured `error_status`.
+ # (defaults to `422 Unprocessable Entity` on new apps,
+ # `200 OK` for compatibility reasons on old apps.)
  # Thus an example like this -
  #
  # respond_to :html, :xml

lib/generators/responders/install_generator.rb

@@ -16,6 +16,10 @@ class ApplicationResponder < ActionController::Responder
  # Redirects resources to the collection path (index action) instead
  # of the resource path (show action) for POST/PUT/DELETE requests.
  # include Responders::CollectionResponder
+
+ # Configure default status codes for responding to errors and redirects.
+ self.error_status = :unprocessable_entity
+ self.redirect_status = :see_other
  end
  RUBY
  end