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.

private Protocol protocolDelegate = Mockito.mock(Protocol.class);
    private Registry registryDelegate = Mockito.mock(Registry.class);
    private Exporter exporter = Mockito.mock(Exporter.class);
    private ServiceConfig<DemoServiceImpl> service = new ServiceConfig<DemoServiceImpl>();
    private ServiceConfig<DemoServiceImpl> service2 = new ServiceConfig<DemoServiceImpl>();
    private ServiceConfig<DemoServiceImpl> delayService = new ServiceConfig<DemoServiceImpl>();
    @BeforeEach
    public void setUp() throws Exception {
        MockProtocol2.delegate = protocolDelegate;
        MockRegistryFactory2.registry = registryDelegate;
        Mockito.when(protocolDelegate.export(Mockito.any(Invoker.class))).thenReturn(exporter);
        ApplicationConfig app = new ApplicationConfig("app");
        ProtocolConfig protocolConfig = new ProtocolConfig();
        protocolConfig.setName("mockprotocol2");
        ProviderConfig provider = new ProviderConfig();
        provider.setExport(true);
        provider.setProtocol(protocolConfig);
        RegistryConfig registry = new RegistryConfig();
        registry.setProtocol("mockprotocol2");
        registry.setAddress("N/A");