When I set `path` in ServiceConfig or `contextpath` in ProtocolConfig, RestProtocol throws NPE on this line:
https://github.com/apache/incubator-dubbo/blob/master/dubbo-rpc/dubbo-rpc-rest/src/main/java/org/apache/dubbo/rpc/protocol/rest/RestProtocol.java#L93
```java
Class implClass = ApplicationModel.getProviderModel(url.getServiceKey()).getServiceInstance().getClass();
```

<dubbo:service path="test" interface="org.apache.dubbo.samples.rest.api.facade.UserRestService" ref="userRestService"
                   protocol="rest" validation="true"/>

This is because we use `group/interface:version` as the key when putting ProviderModel:
```java
Application.initProviderModel(group/interface:version, providerModel)
```
but use `group/path:version` to get, as showed above.

I think to unify to `path` would solve this problem.

}
        }
        String hostToRegistry = ConfigUtils.getSystemProperty(Constants.DUBBO_IP_TO_REGISTRY);
        if (StringUtils.isEmpty(hostToRegistry)) {
            hostToRegistry = NetUtils.getLocalHost();
        }
        map.put(Constants.REGISTER_IP_KEY, hostToRegistry);
        ref = createProxy(map);
        ApplicationModel.initConsumerModel(getUniqueServiceName(), buildConsumerModel(attributes));
    }
    private ConsumerModel buildConsumerModel(Map<String, Object> attributes) {
        Method[] methods = interfaceClass.getMethods();
        Class serviceInterface = interfaceClass;
        if (interfaceClass == GenericService.class) {
            try {
                serviceInterface = Class.forName(interfaceName);
                methods = serviceInterface.getMethods();
            } catch (ClassNotFoundException e) {
                methods = interfaceClass.getMethods();
            }
        }
        return new ConsumerModel(getUniqueServiceName(), serviceInterface, ref, methods, attributes);
    }
    @SuppressWarnings({"unchecked", "rawtypes", "deprecation"})
    private T createProxy(Map<String, String> map) {
        if (shouldJvmRefer(map)) {
            URL url = new URL(Constants.LOCAL_PROTOCOL, Constants.LOCALHOST_VALUE, 0, interfaceClass.getName()).addParameters(map);
            invoker = refprotocol.refer(interfaceClass, url);
            if (logger.isInfoEnabled()) {
                logger.info("Using injvm service " + interfaceClass.getName());