Undefine Rails' assets method before redefining it, to avoid warnings

Rails test suite keeps raising:

    warning: method redefined; discarding old assets
    warning: method redefined; discarding old assets=

Because sprockets-rails is monkey-patching Rails::Application, redefining
the assets method, and adding an attr_writer for it.

This undefines the existing method before adding the new one, avoiding
the warning. The writer call is also removed, since Rails defines an
attr_accessor that already adds it.

Author: carlosantoniodasilva

lib/sprockets/railtie.rb

@@ -14,6 +14,8 @@ class Configuration
       end
     end
 
+    # Undefine Rails' assets method before redefining it, to avoid warnings.
+    undef_method :assets
     # Returns Sprockets::Environment for app config.
     def assets
       return @assets if defined? @assets
@@ -29,7 +31,6 @@ def assets
         end
       end
     end
-    attr_writer :assets
   end
 end