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.

doExport();
        }
    }
    private boolean shouldExport() {
        Boolean shouldExport = getExport();
        if (shouldExport == null && provider != null) {
            shouldExport = provider.getExport();
        }
        // 
        if (shouldExport == null) {
            return true;
        }
        return shouldExport;
    }
    private boolean shouldDelay() {
        Integer delay = getDelay();
        if (delay == null && provider != null) {
            delay = provider.getDelay();
        }
        return delay != null && delay > 0;
    }
    protected synchronized void doExport() {
        if (unexported) {
            throw new IllegalStateException("The service " + interfaceClass.getName() + " has already unexported!");
        }
        if (exported) {
            return;
        }
        exported = true;
        if (StringUtils.isEmpty(path)) {