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 URL getRegisteredProviderUrl(final URL providerUrl, final URL registryUrl) {
        
        if (!registryUrl.getParameter(SIMPLIFIED_KEY, false)) {
            return providerUrl.removeParameters(getFilteredKeys(providerUrl)).removeParameters(
                    MONITOR_KEY, BIND_IP_KEY, BIND_PORT_KEY, QOS_ENABLE, QOS_PORT, ACCEPT_FOREIGN_IP, VALIDATION_KEY,
                    INTERFACES);
        } else {
            String[] paramsToRegistry = getParamsToRegistry(DEFAULT_REGISTER_PROVIDER_KEYS,
                    registryUrl.getParameter(EXTRA_KEYS_KEY, new String[0]));
            return URL.valueOf(providerUrl, paramsToRegistry, providerUrl.getParameter(METHODS_KEY, (String[]) null));
        }
    }
    private URL getSubscribedOverrideUrl(URL registeredProviderUrl) {
        return registeredProviderUrl.setProtocol(PROVIDER_PROTOCOL)
                .addParameters(CATEGORY_KEY, CONFIGURATORS_CATEGORY, CHECK_KEY, String.valueOf(false));
    }
    /**
     * Get the address of the providerUrl through the url of the invoker
     *
     * @param originInvoker
     * @return
     */
    private URL getProviderUrl(final Invoker<?> originInvoker) {
        String export = originInvoker.getUrl().getParameterAndDecoded(EXPORT_KEY);
        if (export == null || export.length() == 0) {
            throw new IllegalArgumentException("The registry export url is null! registry: " + originInvoker.getUrl());
        }
        return URL.valueOf(export);
    }