fix: make config.assets.precompile overridable again

In previous versions it was possible to set (instead of extend) the `precompile` array.
This was broken, seemingly by accident, in rails@af64798

fixes rails#218
fixes rails#327
fixes rails#390

Author: eins78

lib/sprockets/railtie.rb

@@ -103,9 +103,9 @@ 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 )
+        app.config.assets.precompile = %w( manifest.js )
       else
-        app.config.assets.precompile += [LOOSE_APP_ASSETS, /(?:\/|\\|\A)application\.(css|js)$/]
+        app.config.assets.precompile = [LOOSE_APP_ASSETS, /(?:\/|\\|\A)application\.(css|js)$/]
       end
     end