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.

public void testGeneric2() throws Exception {
        Assertions.assertThrows(IllegalArgumentException.class, () -> {
            ServiceConfig service = new ServiceConfig();
            service.setGeneric("illegal");
        });
    }
    @Test
    public void testMock() throws Exception {
        Assertions.assertThrows(IllegalArgumentException.class, () -> {
            ServiceConfig service = new ServiceConfig();
            service.setMock("true");
        });
    }
    @Test
    public void testMock2() throws Exception {
        Assertions.assertThrows(IllegalArgumentException.class, () -> {
            ServiceConfig service = new ServiceConfig();
            service.setMock(true);
        });
    }
    @Test
    public void testUniqueServiceName() throws Exception {
        ServiceConfig<Greeting> service = new ServiceConfig<Greeting>();
        service.setGroup("dubbo");
        service.setInterface(Greeting.class);
        service.setVersion("1.0.0");
        assertThat(service.getUniqueServiceName(), equalTo("dubbo/" + Greeting.class.getName() + ":1.0.0"));
    }
}