Fix Error 500 when searching Wiki pages

If a Wiki page turns up a hit in the search results, an error will occur:

```
Completed 500 Internal Server Error in 836ms

NoMethodError - undefined method `slug' for "test.markdown":String:
  app/helpers/wiki_helper.rb:10:in `namespace_project_wiki_path'
  app/views/search/results/_wiki_blob.html.haml:4:in `_app_views_search_results__wiki_blob_html_haml___2752621660395393333_70299911622700'
  actionview (4.1.9) lib/action_view/template.rb:145:in `block in render'
  activesupport (4.1.9) lib/active_support/notifications.rb:161:in `instrument'
  actionview (4.1.9) lib/action_view/template.rb:339:in `instrument'
  actionview (4.1.9) lib/action_view/template.rb:143:in `render'
```

An unhandled String containing the name of the Wiki page would be provided to the URL path generator. This MR handles that case.

Closes #1547

See merge request !592