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("Unexpected error occured when unexport " + exporter, t);
                }
            }
            exporters.clear();
        }
        unexported = true;
    }
    @SuppressWarnings({"unchecked", "rawtypes"})
    private void doExportUrls() {
        List<URL> registryURLs = loadRegistries(true);
        for (ProtocolConfig protocolConfig : protocols) {
            doExportUrlsFor1Protocol(protocolConfig, registryURLs);
        }
    }
    private void doExportUrlsFor1Protocol(ProtocolConfig protocolConfig, List<URL> registryURLs) {
        String name = protocolConfig.getName();
        if (StringUtils.isEmpty(name)) {
            name = Constants.DUBBO;
        }
        Map<String, String> map = new HashMap<String, String>();
        map.put(Constants.SIDE_KEY, Constants.PROVIDER_SIDE);
        appendRuntimeParameters(map);
        appendParameters(map, application);
        appendParameters(map, module);
        appendParameters(map, provider, Constants.DEFAULT_KEY);
        appendParameters(map, protocolConfig);
        appendParameters(map, this);
        if (CollectionUtils.isNotEmpty(methods)) {
            for (MethodConfig method : methods) {
                appendParameters(map, method, method.getName());