@SentinelRestTemplate注解未生效

## Issue Description

@SentinelRestTemplate 注解未生效

请求的地址服务端关闭后，测试请求抛异常退出了，并没有走熔断处理
抛出异常

```
java.lang.NullPointerException
at org.springframework.web.client.DefaultResponseErrorHandler.hasError(DefaultResponseErrorHandler.java:55)
at org.springframework.web.client.RestTemplate.handleResponse(RestTemplate.java:766)
at org.springframework.web.client.RestTemplate.doExecute(RestTemplate.java:736)
at org.springframework.web.client.RestTemplate.execute(RestTemplate.java:670)
at org.springframework.web.client.RestTemplate.postForObject(RestTemplate.java:414)
```

如果去掉@SentinelRestTemplate，抛出 Connection timed out异常

### Describe what happened (or what feature you want)

- 配置类

```java
@Configuration
public class RestClientConfig {

@Bean
@SentinelRestTemplate(blockHandlerClass = FuseHandlerUtil.class, fallbackClass = FuseHandlerUtil.class,
blockHandler = "blockHandleException", fallback = "fallBackHandleException")
public RestTemplate restTemplate() {
return new RestTemplate();
}

}
```

- resttemplate封装

```java
@Component
public class RestClient {

@Autowired
private RestTemplate restTemplate;

public String sentPost() {
String resp = restTemplate.postForObject("http://localhost:8080/hello/normal",null,
String.class, new Object());
return resp;
}
}
```
- 熔断处理

```java
public final class FuseHandlerUtil {

private static Logger logger = LoggerFactory.getLogger(FuseHandlerUtil.class);

private FuseHandlerUtil(){}

public static ClientHttpResponse blockHandleException(HttpRequest request, byte[] body,
ClientHttpRequestExecution execution, BlockException exception) {
logger.info("触发了限流处理");
try {
return execution.execute(request, body);
} catch (IOException e) {
logger.info("限流处理失败");
logger.error(e.getMessage());
return null;
}
}

public static ClientHttpResponse fallBackHandleException(HttpRequest request, byte[] body,
ClientHttpRequestExecution execution, BlockException exception) {
logger.info("触发了降级处理");
try {
return execution.execute(request, body);
} catch (IOException e) {
logger.info("降级处理失败");
logger.error(e.getMessage());
return null;
}
}
}
```

- 测试类

```java
@SpringBootTest
@RunWith(SpringRunner.class)
public class RestClientTest {

Logger logger = LoggerFactory.getLogger(RestClientTest.class);

@Autowired
private RestClient restClient;

@Test
public void clientTest() throws InterruptedException {
logger.info("客户端测试");
for(int i = 0; i < 100; i++) {
String rs = restClient.sentPost();
logger.info("result:{}", rs);
Thread.sleep(1000);
}
}
}
```

### Tell us your environment

jdk1.8
spring boot 2.1.6.RELEASE
spring web 5.1.8.RELEASE
sentinel 2.1.0.RELEASE

Reproduce the failure from RestClientTest using the RestClientConfig bean and its @SentinelRestTemplate annotation, with the unavailable localhost service described in the issue. Trace why the configured block and fallback handlers do not handle the request error; done means the request reaches the expected fallback or circuit-breaking behavior instead of terminating with the shown exception.

Written by the indexing model from the issue text.