superfluous response.WriteHeader call

Using:

```
:2015
fastcgi / 127.0.0.1:9000 php {
connect_timeout 120s
read_timeout 120s
}
log stdout
tls off
errors
on startup php-fpm7 # php variant only
```

I see:

```
07/May/2019:01:13:50 +0000 [ERROR 502 /Manage_Units_list.php] write tcp 172.17.0.2:2015->172.17.0.1:42698: write: broken pipe
2019/05/07 01:13:50 http: superfluous response.WriteHeader call from github.com/mholt/caddy/caddyhttp/httpserver.(*ResponseRecorder).WriteHeader (recorder.go:61)
```

Any ideas what this down to?

```
php: Pulling from abiosoft/caddy
Digest: sha256:3761c16205a979cf526b084ce941216a1b8bd8646f4e48dbcdc3096fe44deb0e
Status: Image is up to date for abiosoft/caddy:php

```