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 <T> void reExport(final Invoker<T> originInvoker, URL newInvokerUrl) {
        
        ExporterChangeableWrapper exporter = doChangeLocalExport(originInvoker, newInvokerUrl);
        
        URL registryUrl = getRegistryUrl(originInvoker);
        final URL registeredProviderUrl = getRegisteredProviderUrl(newInvokerUrl, registryUrl);
        
        ProviderInvokerWrapper<T> providerInvokerWrapper = ProviderConsumerRegTable.getProviderWrapper(registeredProviderUrl, originInvoker);
        ProviderInvokerWrapper<T> newProviderInvokerWrapper = ProviderConsumerRegTable.registerProvider(originInvoker, registryUrl, registeredProviderUrl);
        /**
         * Only if the new url going to Registry is different with the previous one should we do unregister and register.
         */
        if (providerInvokerWrapper.isReg() && !registeredProviderUrl.equals(providerInvokerWrapper.getProviderUrl())) {
            unregister(registryUrl, providerInvokerWrapper.getProviderUrl());
            register(registryUrl, registeredProviderUrl);
            newProviderInvokerWrapper.setReg(true);
        }
        exporter.setRegisterUrl(registeredProviderUrl);
    }
    /**
     * Reexport the invoker of the modified url
     *
     * @param originInvoker
     * @param newInvokerUrl
     */
    @SuppressWarnings("unchecked")
    private <T> ExporterChangeableWrapper doChangeLocalExport(final Invoker<T> originInvoker, URL newInvokerUrl) {
        String key = getCacheKey(originInvoker);
        final ExporterChangeableWrapper<T> exporter = (ExporterChangeableWrapper<T>) bounds.get(key);
        if (exporter == null) {