Marshaling Without a Schema

Currently running v0.4.0 and noticing that if you attempt to pass `None` as the first argument to `marshal_with`, you receive a 500 response. Here's the minimum example to reproduce:

```Python
import flask
import flask_apispec

application = flask.Flask(__name__)

BLUEPRINT = flask.Blueprint('users', __name__, url_prefix='/users')

class Resource(flask_apispec.MethodResource):

@flask_apispec.marshal_with(None, 204, 'No Content')
def get(self):
return None, 204

BLUEPRINT.add_url_rule('',
endpoint='get',
view_func=Resource.as_view('get'),
methods=['GET'])

application.register_blueprint(BLUEPRINT)
```

Here's the stack trace that I'm seeing.
```
Traceback (most recent call last):
File "/Users/gkadillak/.virtualenvs/temp-api/lib/python2.7/site-packages/flask/app.py", line 1982, in wsgi_app
response = self.full_dispatch_request()
File "/Users/gkadillak/.virtualenvs/temp-api/lib/python2.7/site-packages/flask/app.py", line 1614, in full_dispatch_request
rv = self.handle_user_exception(e)
File "/Users/gkadillak/.virtualenvs/temp-api/lib/python2.7/site-packages/flask/app.py", line 1517, in handle_user_exception
reraise(exc_type, exc_value, tb)
File "/Users/gkadillak/.virtualenvs/temp-api/lib/python2.7/site-packages/flask/app.py", line 1612, in full_dispatch_request
rv = self.dispatch_request()
File "/Users/gkadillak/.virtualenvs/temp-api/lib/python2.7/site-packages/flask/app.py", line 1598, in dispatch_request
return self.view_functions[rule.endpoint](**req.view_args)
File "/Users/gkadillak/.virtualenvs/temp-api/lib/python2.7/site-packages/flask/views.py", line 84, in view
return self.dispatch_request(*args, **kwargs)
File "/Users/gkadillak/.virtualenvs/temp-api/lib/python2.7/site-packages/flask/views.py", line 149, in dispatch_request
return meth(*args, **kwargs)
File "/Users/gkadillak/.virtualenvs/temp-api/lib/python2.7/site-packages/flask_apispec/annotations.py", line 117, in wrapped
return wrapper(*args, **kwargs)
File "/Users/gkadillak/.virtualenvs/temp-api/lib/python2.7/site-packages/flask_apispec/wrapper.py", line 28, in __call__
return self.marshal_result(unpacked, status_code)
File "/Users/gkadillak/.virtualenvs/temp-api/lib/python2.7/site-packages/flask_apispec/wrapper.py", line 49, in marshal_result
output = schema.dump(unpacked[0]).data
AttributeError: 'dict' object has no attribute 'dump'
```