Detect superfluous response.WriteHeader calls

Multiple calls to `response.WriteHeader` (package `net/http`) are superfluous.

Examples:

```
w.WriteHeader(http.StatusOK)
if _, err := w.Write([]byte("ok")); err != nil {
	w.WriteHeader(http.StatusInternalServerError)
}
```

The second call to `WriteHeader` will issue a log line in [`net/http`](https://golang.org/src/net/http/server.go?#L1122).