Don't exit on eof in header reading in stdcopy

There was random lines drops in docker logs because of this
For example you can reproduce this with next lines:
```
docker run -d --name test ubuntu ping google.com && sleep 10 && docker kill test
for i in $(seq 1 100); do docker logs profit_logstash | wc -l; done
```
@creack said something about nat and boot2docker :) But I'm not very sure what it was.