Allow releasing tags without owner set

```
  File "/var/praekelt/vumi-go/go/channel/view_definition.py", line 49, in post
    channel.release(request.user_api)

  File "/var/praekelt/vumi-go/go/vumitools/channel/models.py", line 27, in release
    user_api.release_tag((self.tagpool, self.tag))

  File "/var/praekelt/vumi-go/ve/src/vumi/vumi/persist/model.py", line 567, in wrapper
    return manager.call_decorator(func)(self, *args, **kw)

  File "/var/praekelt/vumi-go/ve/src/vumi/vumi/utils.py", line 369, in wrapped
    result = gen.send(result)

  File "/var/praekelt/vumi-go/go/vumitools/api.py", line 404, in release_tag
    del tag_info.metadata['user_account']

  File "/var/praekelt/vumi-go/ve/src/vumi/vumi/persist/fields.py", line 466, in __delitem__
    self._descriptor.delete_dynamic_value(self._modelobj, key)

  File "/var/praekelt/vumi-go/ve/src/vumi/vumi/persist/fields.py", line 420, in delete_dynamic_value
    del modelobj._riak_object._data[key]

KeyError: 'metadata.user_account'
```
I don't think ``DynamicProxy`` has ``.pop()``.