Always remove the senddata header

internal/senddata/senddata.go

@@ -48,6 +48,7 @@ func (s *sendDataResponseWriter) WriteHeader(status int) {
 		return
 	}
 
+	s.Header().Del(HeaderKey)
 	s.rw.WriteHeader(s.status)
 }
 

internal/senddata/senddata_test.go

+package senddata
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func TestHeaderDelete(t *testing.T) {
+	for _, code := range []int{200, 400} {
+		recorder := httptest.NewRecorder()
+		rw := &sendDataResponseWriter{rw: recorder, req: &http.Request{}}
+		rw.Header().Set(HeaderKey, "foobar")
+		rw.WriteHeader(code)
+
+		if header := recorder.Header().Get(HeaderKey); header != "" {
+			t.Fatalf("HTTP %d response: expected header to be empty, found %q", code, header)
+		}
+	}
+}