Logspout + AWS ECS: first 8 bytes of log message are always removed

With https://github.com/gliderlabs/logspout/commit/e0d92638b3f8e8eea4c5683498b9364b7e8e9050 you untroduced a "feature" which removes the first 8 Bytes from the log message.

router/pump.go
`
if hasHeaders {
buf.Discard(8)
}
`

Unfortunately hasHeaders will always be true when DockerContainer are running on Amazon ECS (No TTY available when running container on ECS). Due to that logspout will always remove the first 8 bytes from my logstring, which breaks my jsonoutput. I imagine I'm not the only one who uses the combination container-engine + json message + elk stack.

Start in router/pump.go and inspect how hasHeaders is determined before buf.Discard(8) runs. Reproduce the ECS-style no-TTY Docker logging case with JSON output, then verify that valid log messages retain their leading bytes without breaking header handling.

Written by the indexing model from the issue text.