net/netcheck: don't use a space in the captive portal challenge

The derpers don't allow whitespace in the challenge.

Change-Id: I93a8b073b846b87854fba127b5c1d80db205f658
Signed-off-by: Andrew Dunham <andrew@tailscale.com>

cmd/derper/derper_test.go

@@ -85,6 +85,11 @@ func TestNoContent(t *testing.T) {
 			input: "input",
 			want:  "response input",
 		},
+		{
+			name:  "valid challenge hostname",
+			input: "ts_derp99b.tailscale.com",
+			want:  "response ts_derp99b.tailscale.com",
+		},
 		{
 			name:  "invalid challenge",
 			input: "foo\x00bar",