Extension does not respect custom uploader implementations

[ckanext--s3filestore](https://github.com/okfn/ckanext-s3filestore) implements a custom uploader and downloader, [overriding](https://github.com/okfn/ckanext-s3filestore/blob/master/ckanext/s3filestore/plugin.py#L57) the core download endpoint. ckanext-googleanalytics does the same, to register the download and [calls the core download action](https://github.com/ckan/ckanext-googleanalytics/blob/master/ckanext/googleanalytics/controller.py#L142) from the package controller afterwards.

So if you load the `googleanalytics` before the `s3filestore` one, your downloads return 404, because the core filesystem filestore is being called:

```
ckan.plugins = ... googleanalytics s3filestore # Does not work
```

Loading `googleanalytics` last works:

```
ckan.plugins = ... s3filestore googleanalytics # Works
```