Fix empty host for an asset url when asset_host proc returns nil

If a CDN can be dynamically enabled in an app, when it is switched off and the asset_host proc returns nil, all asset urls are relative instead of absolute. This fix makes the urls absolute.
```
assert_equal "/images/rails.png", image_path("rails.png")
```

.. just as a direct contrast to `image_url`, because the other similar test that I saw doesn't set up `base_url`.