Support custom Sprocket manifests

This change adds support for Rails 4 apps that have set their manifest to a
custom one.

In Rails 4, if `config.assets.manifest` is set, it must include the
manifest's filename. For example:

    config.assets.manifest = "public/assets/manifest-foo.json"

Note that while it's invalid for the manifest option to not include the filename
in Rails 4, we are more forgiving and also accept values that don't contain
the filename, in order to preserve backwards compatibility with previous
requirejs-rails versions and Rails 3 apps.

Author: agis

lib/requirejs/rails/engine.rb

@@ -36,7 +36,12 @@ class Engine < ::Rails::Engine
           end
         end
 
-        manifest_directory = config.assets.manifest || File.join(::Rails.public_path, config.assets.prefix)
+        manifest_directory = if config.assets.manifest
+          File.basename(config.assets.manifest) =~ /\./ ? File.dirname(config.assets.manifest) : config.assets.manifest
+        else
+          File.join(::Rails.public_path, config.assets.prefix)
+        end
+
         manifest_path = File.join(manifest_directory, "rjs_manifest.yml")
         config.requirejs.manifest_path = Pathname.new(manifest_path)
       end
@@ -53,8 +58,7 @@ class Engine < ::Rails::Engine
       if ::Rails::VERSION::MAJOR >= 4
         config.after_initialize do |app|
           config = app.config
-          rails_manifest_path = File.join(app.root, 'public', config.assets.prefix)
-          rails_manifest = ::Sprockets::Manifest.new(app.assets, rails_manifest_path)
+          rails_manifest = ::Sprockets::Manifest.new(app.assets, config.assets.manifest)
           if config.requirejs.manifest_path.exist? && rails_manifest
             rjs_digests = YAML.load(ERB.new(File.new(config.requirejs.manifest_path).read).result)
             rails_manifest.assets.merge!(rjs_digests)