Add asset path to asset tag helper

Two tests are failing:

```
[12/93] AssetTagHelperTest#test_asset_path = 0.00 s
  1) Failure:
test_asset_path(AssetTagHelperTest) [template/asset_tag_helper_test.rb:537]:
<"/assets/xml"> expected to be == to
<"//xml">.

[65/93] AssetTagHelperTest#test_path_to_asset_alias_for_asset_path = 0.00 s
  2) Failure:
test_path_to_asset_alias_for_asset_path(AssetTagHelperTest) [template/asset_tag_helper_test.rb:541]:
<"/assets/xml"> expected to be == to
<"//xml">.
```

These two still have to be fixed in order to merge this. My (long and very possibly wrong) guess is that `ActionView::Helpers::AssetTagHelper::AssetPaths` is not figuring out those paths are actually assets.

I also plan to add tests to other assets, as this is basically a copy and paste from `audio_path` and `path_to_asset` tests.