An answer from the app without a Content-Type reaches the client without one (closes #127)

Go's server adds a Content-Type it guesses from the body when the
response's headers have no Content-Type key, which happened to some
of the app's answers and not others, as the first part of the body or
the headers went out first. When the app sends none, smallwebwaf now
sets the key with no value on the client's response, which sends none.

Model: opus-5-5

internal/proxy/passthrough_test.go

@@ -192,6 +192,36 @@ func wantRequestFields(t *testing.T, line logLine, host string, sent, received i
 	}
 }
 
+func TestAddsNoContentTypeTheAppDidNotSend(t *testing.T) {
+	t.Parallel()
+
+	const typed = "text/plain"
+
+	app := startApp(t, func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path == "/typed" {
+			w.Header().Set("Content-Type", typed)
+		} else {
+			// The key with no value keeps Go's server from adding one.
+			w.Header()["Content-Type"] = nil
+		}
+
+		_, _ = io.WriteString(w, "<!doctype html><title>page</title>")
+	})
+	addr, _ := startProxy(t, app.URL, nil)
+
+	got := get(t, addr, "/")
+	if got.header.Values("Content-Type") != nil {
+		t.Errorf("client got Content-Type %q, want none",
+			got.header.Get("Content-Type"))
+	}
+
+	got = get(t, addr, "/typed")
+	if got.header.Get("Content-Type") != typed {
+		t.Errorf("client got Content-Type %q, want %q",
+			got.header.Get("Content-Type"), typed)
+	}
+}
+
 func TestStreamsTheRequestBody(t *testing.T) {
 	t.Parallel()