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.

logger.warn(new IllegalStateException("error state, exporter should not be null"));
        } else {
            final Invoker<T> invokerDelegate = new InvokerDelegate<T>(originInvoker, newInvokerUrl);
            exporter.setExporter(protocol.export(invokerDelegate));
        }
        return exporter;
    }
    /**
     * Get an instance of registry based on the address of invoker
     *
     * @param originInvoker
     * @return
     */
    private Registry getRegistry(final Invoker<?> originInvoker) {
        URL registryUrl = getRegistryUrl(originInvoker);
        return registryFactory.getRegistry(registryUrl);
    }
    private URL getRegistryUrl(Invoker<?> originInvoker) {
        URL registryUrl = originInvoker.getUrl();
        if (REGISTRY_PROTOCOL.equals(registryUrl.getProtocol())) {
            String protocol = registryUrl.getParameter(REGISTRY_KEY, DEFAULT_DIRECTORY);
            registryUrl = registryUrl.setProtocol(protocol).removeParameter(REGISTRY_KEY);
        }
        return registryUrl;
    }
    /**
     * Return the url that is registered to the registry and filter the url parameter once
     *
     * @param providerUrl
     * @return url to registry.