sentinel 1.8.0集成 zuul 1.x， API限流 servletPath匹配问题

## Issue Description
zuul下面挂了多个子系统，各个子系统通过routeId做区分；想通过sentinel对各个子系统不同访问路劲进行限流控制；配置如下：
`private void initCustomizedApis() {
Set apiDefinitions = new HashSet<>();
ApiDefinition userQuery = new ApiDefinition();
userQuery.setApiName("user_query");
Set apiPaths = new HashSet<>();
userQuery.setPredicateItems(apiPaths);

ApiPathPredicateItem api = new ApiPathPredicateItem();
api.setPattern("/zuul/member/query/user");
api.setMatchStrategy(SentinelGatewayConstants.URL_MATCH_STRATEGY_EXACT);
apiPaths.add(api);

apiDefinitions.add(userQuery);
GatewayApiDefinitionManager.loadApiDefinitions(apiDefinitions);
}
`
`private void initGatewayRules() {
Set myRules = new HashSet<>();

GatewayFlowRule userQuery = new GatewayFlowRule();
userQuery.setResourceMode(SentinelGatewayConstants.RESOURCE_MODE_CUSTOM_API_NAME)
.setResource("user_query")
.setCount(2)
.setGrade(RuleConstant.FLOW_GRADE_QPS);

// 添加到规则列表
myRules.add(userQuery);

// 手动加载
GatewayRuleManager.loadRules(myRules);
}`

调试了一下，发现都是通过servletpath去匹配请求路劲，而我的servlet path是 /zuul，每次都是固定的；请问下，你们这个demo是怎么测试通过的？
‘public static Predicate exactPath(final String path) {
return new Predicate() {
@Override
public boolean test(RequestContext exchange) {
return exchange.getRequest().getServletPath().equals(path);
}
};
}’

exchange.getRequest().getServletPath（）固定返回都是 '/zuul'
path: 是我自定义的url路劲

Type: *bug report* or *feature request*

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

### Describe what you expected to happen

### How to reproduce it (as minimally and precisely as possible)

1.
2.
3.

### Tell us your environment

### Anything else we need to know?

Start with the shown exactPath predicate and the gateway API-definition and flow-rule setup, then reproduce the request through Zuul with the `/zuul` servlet path. Trace which request path the matcher receives and verify that the configured custom API is matched for the intended route; done means the rule distinguishes the subsystem URL rather than only the fixed servlet path.

Written by the indexing model from the issue text.