Fix RepositoriesFilesystemControllerTest for jRuby

Iconv on jRuby does not accept an empty string as "from" encoding.
Use the default behavior from the controller instead to leave it
unspecified and thuse use UTF-8.

test/functional/repositories_filesystem_controller_test.rb

@@ -37,7 +37,7 @@ class RepositoriesFilesystemControllerTest < ActionController::TestCase
     @repository = Repository::Filesystem.create(
                       :project => Project.find(PRJ_ID),
                       :url     => REPOSITORY_PATH,
-                      :path_encoding => ''
+                      :path_encoding => nil
                       )
     assert @repository
   end