Engine config: replace #cache_duration_in_days with #cache_duration

README.md

@@ -187,7 +187,7 @@ end
 # Rails.cache. Some cache stores may not automatically purge expired content,
 # such as the default FileStore.
 # http://guides.rubyonrails.org/caching_with_rails.html#cache-stores
-Riiif::Engine.config.cache_duration_in_days = 30
+Riiif::Engine.config.cache_duration = 30.days
 ```
 #### Special note for Passenger and Apache users
 If you are running riiif in Passenger under Apache, you must set the following in your virtual host definition:

app/models/riiif/image.rb

@@ -57,7 +57,14 @@ def info
 
     class << self
       def expires_in
-        Riiif::Engine.config.cache_duration_in_days.days
+        if Riiif::Engine.config.respond_to?(:cache_duration_in_days)
+          Deprecation.warn(self,
+                           'Riiif::Engine.config.cache_duration_in_days is deprecated; '\
+                           'use #cache_duration instead and pass a fully-qualified date (e.g., `3.days`)')
+          Riiif::Engine.config.cache_duration_in_days.days
+        else
+          Riiif::Engine.config.cache_duration
+        end
       end
 
       def cache_key(id, options)

lib/riiif/engine.rb

@@ -4,7 +4,7 @@ class Engine < ::Rails::Engine
     require 'riiif/rails/routes'
 
     # How long to cache the tiles for.
-    config.cache_duration_in_days = 3
+    config.cache_duration = 3.days
 
     config.action_dispatch.rescue_responses['Riiif::ImageNotFoundError'] = :not_found