Fix issue in GlassfishRequestUpgradeStrategy

The observed behavior was that the client does not get a response from
the WebSocket HTTP handshake. On the server the handshake actually
succeeds, the response is set correctly to status 101, and the
WebSocketHandler gets notified of the successfully established
connection.

This change flushes the ServletResponse just before returning from the
GlassfishRequestUpgradeStrategy. This is actually what Glassfish's own
TyrusServletFilter does as well at the end along with a comment that it
is a possible bug.

spring-websocket/src/main/java/org/springframework/web/socket/server/standard/GlassFishRequestUpgradeStrategy.java

@@ -163,21 +163,27 @@ public class GlassFishRequestUpgradeStrategy extends AbstractStandardUpgradeStra
 
 		Connection connection = createConnection(upgradeHandler, response);
 
-		RequestContext wsRequest = RequestContext.Builder.create().
+		RequestContext requestContext = RequestContext.Builder.create().
 				requestURI(URI.create(wsApp.getPath())).requestPath(wsApp.getPath()).
 				userPrincipal(request.getUserPrincipal()).
 				connection(connection).secure(request.isSecure()).build();
 
 		for (String header : headers.keySet()) {
-			wsRequest.getHeaders().put(header, headers.get(header));
+			requestContext.getHeaders().put(header, headers.get(header));
 		}
 
-		return WebSocketEngine.getEngine().upgrade(connection, wsRequest, new WebSocketEngine.WebSocketHolderListener() {
-			@Override
-			public void onWebSocketHolder(WebSocketEngine.WebSocketHolder webSocketHolder) {
-				upgradeHandler.setWebSocketHolder(webSocketHolder);
-			}
-		});
+		boolean upgraded = WebSocketEngine.getEngine().upgrade(connection, requestContext,
+				new WebSocketEngine.WebSocketHolderListener() {
+					@Override
+					public void onWebSocketHolder(WebSocketEngine.WebSocketHolder webSocketHolder) {
+						upgradeHandler.setWebSocketHolder(webSocketHolder);
+					}
+				});
+
+		// Glassfish bug ?? (see same line in TyrusServletFilter.doFilter)
+		response.flushBuffer();
+
+		return upgraded;
 	}
 
 	private WebSocketApplication createTyrusEndpoint(HttpServletRequest request,