Allow using a custom assets manifest via app config

At the moment the only allowed path is the default one but
there is a config.assets.manifest option that should set
the path of the custom manifest path.

When an application uses a custom location for the manifest,
even by setting the path like:

```
config.assets.manifest = File.expand_path('my_custom_path/config/manifest.js', __dir__)
```

would raise a ManifestNeededError exception.

This commit adds a check of the configured assets manifest
path before trying to use the default location.

lib/sprockets/railtie.rb

@@ -69,7 +69,11 @@ class Railtie < ::Rails::Railtie
 
     class ManifestNeededError < StandardError
       def initialize
-        msg = "Expected to find a manifest file in `app/assets/config/manifest.js`\n" +
+        manifest_path = Pathname.new(
+          ::Rails.application.config.assets.manifest ||
+          ::Rails.root.join("app/assets/config/manifest.js")
+        )
+        msg = "Expected to find a manifest file in `#{manifest_path}`\n" +
         "But did not, please create this file and use it to link any assets that need\n" +
         "to be rendered by your app:\n\n" +
         "Example:\n" +
@@ -102,8 +106,13 @@ def configure(&block)
 
     initializer :set_default_precompile do |app|
       if using_sprockets4?
-        raise ManifestNeededError unless ::Rails.root.join("app/assets/config/manifest.js").exist?
-        app.config.assets.precompile += %w( manifest.js )
+        manifest_path = Pathname.new(
+          app.config.assets.manifest ||
+          ::Rails.root.join("app/assets/config/manifest.js")
+        )
+        raise ManifestNeededError unless manifest_path.exist?
+
+        app.config.assets.precompile += [manifest_path.to_s]
       else
         app.config.assets.precompile += [LOOSE_APP_ASSETS, /(?:\/|\\|\A)application\.(css|js)$/]
       end

test/test_railtie.rb

@@ -296,8 +296,16 @@ def test_sprockets_context_helper
   end
 
   def test_manifest_path
+    Dir.chdir(app.root) do
+      dir = "app/assets/config/foo"
+      FileUtils.mkdir_p(dir)
+      File.open("#{ dir }/bar.json", "w") do |f|
+        f << ""
+      end
+    end
+
     app.configure do
-      config.assets.manifest = Rails.root.join('config','foo','bar.json')
+      config.assets.manifest = Rails.root.join('app','assets','config','foo','bar.json')
     end
     app.initialize!