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.

delayService.export();
        assertTrue(delayService.getExportedUrls().isEmpty());
        
        TimeUnit.MILLISECONDS.sleep(delayService.getDelay() + 300);
        assertThat(delayService.getExportedUrls(), hasSize(1));
    }
    @Test
    @Disabled("cannot pass in travis")
    public void testUnexport() throws Exception {
        System.setProperty(Constants.SHUTDOWN_WAIT_KEY, "0");
        try {
            service.export();
            service.unexport();
            Thread.sleep(1000);
            Mockito.verify(exporter, Mockito.atLeastOnce()).unexport();
        } finally {
            System.clearProperty(Constants.SHUTDOWN_TIMEOUT_KEY);
        }
    }
    @Test
    public void testInterfaceClass() throws Exception {
        ServiceConfig<Greeting> service = new ServiceConfig<Greeting>();
        service.setInterface(Greeting.class.getName());
        service.setRef(Mockito.mock(Greeting.class));
        assertThat(service.getInterfaceClass() == Greeting.class, is(true));
        service = new ServiceConfig<Greeting>();
        service.setRef(Mockito.mock(Greeting.class, withSettings().extraInterfaces(GenericService.class)));
        assertThat(service.getInterfaceClass() == GenericService.class, is(true));
    }
    @Test