Make Resource methods not have to return a tuple

Because our MethodViews use this pattern:

```python
return self.response_handler.make_item_response(
*self.resource.create(*args, **kwargs)
)
```

self.resource.create needs to return a tuple. We should mimic Flask further and allow single value OR tuple.

Without this change, the developer needs to return a tuple in their Resource method which might lead to code like:

```python
def get(self):
return [],
```

just to satisfy that need. The comma could subsequently be deleted by accident and subtly break the code.