ARI: Fix WebSocket response when subprotocol isn't specified

When I moved the ARI WebSocket from /ws to /ari/events, I added code to
allow a WebSocket to connect without specifying the subprotocol if
there's only one subprotocol handler registered for the WebSocket.

Naively, I coded it to always respond with the subprotocol in use.
Unfortunately, according to RFC 6455, if the server's response includes
a subprotocol header field that "indicates the use of a subprotocol that
was not present in the client's handshake [...], the client MUST _Fail
the WebSocket Connection_.", emphasis theirs.

This patch correctly omits the Sec-WebSocket-Protocol if one is not
specified by the client.

(closes issue ASTERISK-22441)
Review: https://reviewboard.asterisk.org/r/2828/
........

Merged revisions 399039 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@399042 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/res_http_websocket.c

@@ -645,11 +645,25 @@ int AST_OPTIONAL_API_NAME(ast_websocket_uri_cb)(struct ast_tcptls_session_instan
 		fprintf(ser->f, "HTTP/1.1 101 Switching Protocols\r\n"
 			"Upgrade: %s\r\n"
 			"Connection: Upgrade\r\n"
-			"Sec-WebSocket-Accept: %s\r\n"
-			"Sec-WebSocket-Protocol: %s\r\n\r\n",
+			"Sec-WebSocket-Accept: %s\r\n",
 			upgrade,
-			base64,
-			protocol_handler->name);
+			base64);
+
+		/* RFC 6455, Section 4.1:
+		 *
+		 * 6. If the response includes a |Sec-WebSocket-Protocol| header
+		 *    field and this header field indicates the use of a
+		 *    subprotocol that was not present in the client's handshake
+		 *    (the server has indicated a subprotocol not requested by
+		 *    the client), the client MUST _Fail the WebSocket
+		 *    Connection_.
+		 */
+		if (protocol) {
+			fprintf(ser->f, "Sec-WebSocket-Protocol: %s\r\n",
+				protocol);
+		}
+
+		fprintf(ser->f, "\r\n");
 	} else {
 
 		/* Specification defined in http://tools.ietf.org/html/draft-hixie-thewebsocketprotocol-75 or completely unknown */