return correct etag for files requested with non-digest URI

There is a bug in the current release of asset-pipeline so that the etag that it returns when you ask for a file using it's "regular" URI (ex: `asset_test.js`) is just an echo of that filename:

```
ETag:asset_test.js
```

This is bad because it means that the client won't get updated versions of files when they're updated.

This PR fixes this behavior (and adds tests) so that the returned etag is actually the digest name that's been looked up in the manifest rather than just an echo of the filename:

```
ETag:asset_test-b384266fad57f2e383d5d007a35b6fb9.js
```

I also upgraded the `dummy` test project up to grails 2.3.8 (the same version in the main plugin), and bumped up the plugin version to 1.9.3.