Can't download resources with geojson extension

The problem is that ```mimetypes.guess_type``` in
https://github.com/ckan/ckan/blob/master/ckan/controllers/package.py#L1238-L1239
doesn't recognize the ```.geojson``` extension and returns None.

If you search in demo using
http://demo.ckan.org/api/3/action/resource_search?query=format:geojson, any
file with a ```.geojson``` extension won't work, but the ones with ```.json```
will work just fine.

It doesn't break when running with paster, as it doesn't validate the
Content-Type and will simply serve the file with ```Content-Type: None```.

Just for the record, the error message is:

```
[Fri Feb 21 17:58:30 2014] [error] [client 127.0.0.1] mod_wsgi (pid=30210): Exception occurred processing WSGI script '/etc/ckan/demo/apache.wsgi'.
[Fri Feb 21 17:58:30 2014] [error] [client 127.0.0.1] TypeError: expected byte string object for header value, value of type NoneType found
```