fix cache key generation for complex object

In Rails > 4, cache key would be expand fragment name with digest.

https://github.com/rails/rails/blob/4-0-stable/actionpack/lib/action_view/helpers/cache_helper.rb#L115
https://github.com/rails/rails/blob/4-0-stable/actionpack/lib/action_controller/caching/fragments.rb#L21-23

lib/jbuilder/jbuilder_template.rb

@@ -98,6 +98,13 @@ def _render_partial(options)
   end
 
   def _cache_key(key, options)
+    key = fragment_name_with_digest(key, options)
+    ::ActiveSupport::Cache.expand_cache_key(key.is_a?(::Hash) ? url_for(key).split('://').last : key, :jbuilder)
+  end
+
+  private
+
+  def fragment_name_with_digest(key, options)
     if @context.respond_to?(:cache_fragment_name)
       # Current compatibility, fragment_name_with_digest is private again and cache_fragment_name
       # should be used instead.
@@ -106,12 +113,10 @@ def _cache_key(key, options)
       # Backwards compatibility for period of time when fragment_name_with_digest was made public.
       @context.fragment_name_with_digest(key)
     else
-      ::ActiveSupport::Cache.expand_cache_key(key.is_a?(::Hash) ? url_for(key).split('://').last : key, :jbuilder)
+      key
     end
   end
 
-  private
-
   def _mapable_arguments?(value, *args)
     return true if super
     options = args.last

test/jbuilder_template_test.rb

@@ -246,6 +246,7 @@ def assert_collection_rendered(json, context = nil)
     undef_context_methods :fragment_name_with_digest
 
     @context.expects :cache_fragment_name
+    ActiveSupport::Cache.expects :expand_cache_key
 
     render_jbuilder <<-JBUILDER
       json.cache! 'cachekey' do
@@ -258,6 +259,7 @@ def assert_collection_rendered(json, context = nil)
     undef_context_methods :fragment_name_with_digest
 
     @context.expects(:cache_fragment_name).with('cachekey', skip_digest: true)
+    ActiveSupport::Cache.expects :expand_cache_key
 
     render_jbuilder <<-JBUILDER
       json.cache! 'cachekey', skip_digest: true do
@@ -277,16 +279,4 @@ def assert_collection_rendered(json, context = nil)
     assert_equal Rails.cache.inspect[/entries=(\d+)/, 1], '0'
   end
 
-  test 'fragment caching falls back on ActiveSupport::Cache.expand_cache_key' do
-    undef_context_methods :fragment_name_with_digest, :cache_fragment_name
-
-    ActiveSupport::Cache.expects :expand_cache_key
-
-    render_jbuilder <<-JBUILDER
-      json.cache! 'cachekey' do
-        json.name 'Cache'
-      end
-    JBUILDER
-  end
-
 end