marshal_with not working with return code

works perfectly:

```python
@marshal_with(CreditCardResponseSchema)
def create_card():
card = CreditCard()
return card
```

returns an error:

```python
@marshal_with(CreditCardResponseSchema, code=201)
def create_card():
card = CreditCard()
return card
```

```
Traceback (most recent call last):
File "/Users/john/.virtualenvs/real_state_app/lib/python3.6/site-packages/flask/app.py", line 1997, in __call__
return self.wsgi_app(environ, start_response)
File "/Users/john/.virtualenvs/real_state_app/lib/python3.6/site-packages/flask/app.py", line 1985, in wsgi_app
response = self.handle_exception(e)
File "/Users/john/.virtualenvs/real_state_app/lib/python3.6/site-packages/flask/app.py", line 1540, in handle_exception
reraise(exc_type, exc_value, tb)
File "/Users/john/.virtualenvs/real_state_app/lib/python3.6/site-packages/flask/_compat.py", line 33, in reraise
raise value
File "/Users/john/.virtualenvs/real_state_app/lib/python3.6/site-packages/flask/app.py", line 1982, in wsgi_app
response = self.full_dispatch_request()
File "/Users/john/.virtualenvs/real_state_app/lib/python3.6/site-packages/flask/app.py", line 1614, in full_dispatch_request
rv = self.handle_user_exception(e)
File "/Users/john/.virtualenvs/real_state_app/lib/python3.6/site-packages/flask/app.py", line 1517, in handle_user_exception
reraise(exc_type, exc_value, tb)
File "/Users/john/.virtualenvs/real_state_app/lib/python3.6/site-packages/flask/_compat.py", line 33, in reraise
raise value
File "/Users/john/.virtualenvs/real_state_app/lib/python3.6/site-packages/flask/app.py", line 1612, in full_dispatch_request
rv = self.dispatch_request()
File "/Users/john/.virtualenvs/real_state_app/lib/python3.6/site-packages/flask/app.py", line 1598, in dispatch_request
return self.view_functions[rule.endpoint](**req.view_args)
File "/Users/john/.virtualenvs/real_state_app/lib/python3.6/site-packages/flask_jwt/__init__.py", line 177, in decorator
return fn(*args, **kwargs)
File "/Users/john/.virtualenvs/real_state_app/lib/python3.6/site-packages/flask_apispec/annotations.py", line 117, in wrapped
return wrapper(*args, **kwargs)
File "/Users/john/.virtualenvs/real_state_app/lib/python3.6/site-packages/flask_apispec/wrapper.py", line 34, in __call__
return self.marshal_result(unpacked, status_code)
File "/Users/john/.virtualenvs/real_state_app/lib/python3.6/site-packages/flask_apispec/wrapper.py", line 62, in marshal_result
return format_output((format_response(output), ) + unpacked[1:])
File "/Users/john/.virtualenvs/real_state_app/lib/python3.6/site-packages/flask/json.py", line 263, in jsonify
(dumps(data, indent=indent, separators=separators), '\n'),
File "/Users/john/.virtualenvs/real_state_app/lib/python3.6/site-packages/flask/json.py", line 123, in dumps
rv = _json.dumps(obj, **kwargs)
File "/usr/local/Cellar/python/3.6.4_4/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/__init__.py", line 238, in dumps
**kw).encode(obj)
File "/usr/local/Cellar/python/3.6.4_4/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/encoder.py", line 201, in encode
chunks = list(chunks)
File "/usr/local/Cellar/python/3.6.4_4/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/encoder.py", line 437, in _iterencode
o = _default(o)
File "/Users/john/.virtualenvs/real_state_app/lib/python3.6/site-packages/flask/json.py", line 80, in default
return _json.JSONEncoder.default(self, o)
File "/usr/local/Cellar/python/3.6.4_4/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/encoder.py", line 180, in default
o.__class__.__name__)
TypeError: Object of type 'CreditCard' is not JSON serializable
```

Reproduce the two create_card examples, comparing marshal_with with and without code=201. Start in flask_apispec/wrapper.py at marshal_result and inspect how the returned CreditCard is formatted before Flask jsonify runs; done means the response is serialized with the requested status code without the JSON serialization error.

Written by the indexing model from the issue text.