A little bit of info if someone wants to look into this more. I put a breakpoint in `register_service_views` and whether the test passes or fails, `service.definitions` always has the `text/plain` definition first. So that is not the problem I think.

```
$ .tox/py33/bin/nosetests cornice.tests.test_validation:TestServiceDefinition.test_accept
> /Users/marca/dev/git-repos/cornice/cornice/pyramidhook.py(139)register_service_views()
    138 
--> 139     for method, view, args in service.definitions:
    140 

ipdb> service.definitions
[('GET', <function get2 at 0x102250e60>, {'cors_headers': [], 'accept': 'text/plain', ...
```