Sending a request with a body crashes if the server responds before the body is sent

if the server handler is something like
```python
async def handler(request):
    return web.Response(text='ok')
```

then `session.post('…', data=b'0')` will crash (trying to write to a closed transport)

I'll submit a pull request for this next week (i have a memory leak i need to track down first)