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.

Assertions.assertThrows(IllegalArgumentException.class, () -> {
            ServiceBuilder builder = new ServiceBuilder();
            builder.mock("true");
        });
    }
    @Test
    public void Mock1() throws Exception {
        Assertions.assertThrows(IllegalArgumentException.class, () -> {
            ServiceBuilder builder = new ServiceBuilder();
            builder.mock(true);
        });
    }
    @Test
    void build() {
        MethodConfig method = new MethodConfig();
        ProviderConfig provider = new ProviderConfig();
        ServiceBuilder builder = new ServiceBuilder();
        builder.path("path").addMethod(method).provider(provider).providerIds("providerIds")
                .generic(GENERIC_SERIALIZATION_DEFAULT);
        ServiceConfig config = builder.build();
        ServiceConfig config2 = builder.build();
        assertThat(config.getGeneric(), equalTo(GENERIC_SERIALIZATION_DEFAULT));
        Assertions.assertEquals("path", config.getPath());
        Assertions.assertEquals("providerIds", config.getProviderIds());
        Assertions.assertSame(provider, config.getProvider());
        Assertions.assertTrue(config.getMethods().contains(method));
        Assertions.assertEquals(1, config.getMethods().size());
        Assertions.assertNotSame(config, config2);
    }
}