Access logs not properly displaying body size

In certain circumstances, the access logger is displaying the wrong body size.

```python
@app.post("/")
async def post(request):
    print(request.json)
```
```
$ curl localhost:9999 -d '{"foo": "bar"}
```
```
{'foo': 'bar'}
[2021-04-18 01:43:33 +0300] - (sanic.access)[INFO][127.0.0.1:52172]: POST http://localhost:9999/  200 -1
```

In this example, it is being reported as `-1`