twentyone: set xhr header so we get errors back as JSON

files/templates/casino/blackjack_screen.html

@@ -4,6 +4,7 @@
 <script>
 	function makeBlackjackRequest(action) {
 		const xhr = new XMLHttpRequest();
+		xhr.setRequestHeader('xhr', 'xhr');
 		xhr.open("post", `/casino/twentyone/${action}`);
 		xhr.onload = handleBlackjackResponse.bind(null, xhr);
 		xhr.blackjackAction = action;