Fix client-library-url ignored in MVC namespace

Prior to this commit, the `client-library-url` XML attribute was not
effective in the MVC namespace, leaving the default value configured:

```xml
<websocket:sockjs client-library-url="/js/sockjs.js" />
```

This commit fixes the sockjs namespace handler and makes sure that this
attribute is configured on the `SockJsService` Bean to be created.

Issue: SPR-12874

spring-websocket/src/test/java/org/springframework/web/socket/config/HandlersBeanDefinitionParserTests.java

@@ -230,6 +230,7 @@ public class HandlersBeanDefinitionParserTests {
 		assertEquals(256, transportService.getDisconnectDelay());
 		assertEquals(1024, transportService.getHttpMessageCacheSize());
 		assertEquals(20, transportService.getHeartbeatTime());
+		assertEquals("/js/sockjs.min.js", transportService.getSockJsClientLibraryUrl());
 		assertEquals(TestMessageCodec.class, transportService.getMessageCodec().getClass());
 
 		List<HandshakeInterceptor> interceptors = transportService.getHandshakeInterceptors();