Kafka producer lost headers for message

Sometimes AIOKafkaProducer writes messages without headers.

Environment:
- python version `3.7.4`
- aiokafka version `0.5.2`
- kafka-python version `1.4.6`
- docker image `confluentinc/cp-kafka:5.2.1`

In new environment I created a new topic `test` and launched code in Jupyter:
```python
import aiokafka
consumer = aiokafka.AIOKafkaConsumer(
'test',
loop=asyncio.get_event_loop(),
bootstrap_servers='localhost:9092',
)
await consumer.start()
producer = aiokafka.AIOKafkaProducer(
loop=asyncio.get_event_loop(),
bootstrap_servers='localhost:9092',
)
await producer.start()

msg = ('a'*100).encode()
for _ in range(500):
await producer.send('test', value=msg, headers=[('type', b'Normal')])

for _ in range(500):
doc = await consumer.getone()
assert doc.headers == (('type', b'Normal'),)
```
I got AssertionError at ~135th message. Kafka Tool also shows message without headers.
Interesting that producing smaller amount of messages does not raise an error.
Tested also with other Kafka entities and got same result.