Added sample code that shows negotation of extensions. JDK client doe…

…s not support adding the extensions header manually so only server-side code updated.

nima/tests/integration/websocket/server/src/main/java/io/helidon/nima/tests/integration/websocket/webserver/EchoService.java

@@ -43,16 +43,24 @@ public void onClose(WsSession session, int status, String reason) {
 
     @Override
     public Optional<Headers> onHttpUpgrade(HttpPrologue prologue, Headers headers) throws WsUpgradeException {
+        WritableHeaders<?> upgradeHeaders = WritableHeaders.create();
         if (headers.contains(WsUpgradeProvider.PROTOCOL)) {
             List<String> subProtocols = headers.get(WsUpgradeProvider.PROTOCOL).allValues(true);
             if (subProtocols.contains("chat")) {
-                Headers upgradeHeaders = WritableHeaders.create().set(WsUpgradeProvider.PROTOCOL, "chat");
-                return Optional.of(upgradeHeaders);     // negotiated
+                upgradeHeaders.set(WsUpgradeProvider.PROTOCOL, "chat");
             } else {
                 throw new WsUpgradeException("Unable to negotiate WS sub-protocol");
             }
         }
-        return Optional.empty();
+        if (headers.contains(WsUpgradeProvider.EXTENSIONS)) {
+            List<String> extensions = headers.get(WsUpgradeProvider.EXTENSIONS).allValues(true);
+            if (extensions.contains("nima")) {
+                upgradeHeaders.set(WsUpgradeProvider.EXTENSIONS, "nima");
+            } else {
+                throw new WsUpgradeException("Unable to negotiate WS extensions");
+            }
+        }
+        return upgradeHeaders.size() > 0 ? Optional.of(upgradeHeaders) : Optional.empty();
     }
 
     void resetClosed() {

nima/tests/integration/websocket/server/src/test/java/io/helidon/nima/tests/integration/websocket/webserver/WebSocketTest.java

@@ -82,6 +82,7 @@ void testOnce() throws Exception {
 
         java.net.http.WebSocket ws = client.newWebSocketBuilder()
                 .subprotocols("chat", "mute")
+                // .header(EXTENSIONS.defaultCase(), "nima") rejected by client
                 .buildAsync(URI.create("ws://localhost:" + port + "/echo"), listener)
                 .get(5, TimeUnit.SECONDS);
         assertThat(ws.getSubprotocol(), is("chat"));    // negotiated

nima/websocket/webserver/src/main/java/io/helidon/nima/websocket/webserver/WsUpgradeProvider.java

@@ -61,6 +61,11 @@ public class WsUpgradeProvider implements Http1UpgradeProvider {
      */
     public static final HeaderName PROTOCOL = Header.create("Sec-WebSocket-Protocol");
 
+    /**
+     * Websocket protocol header name.
+     */
+    public static final HeaderName EXTENSIONS = Header.create("Sec-WebSocket-Extensions");
+
     /**
      * Switching response prefix.
      */