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.

if (i >= 0 && i < url.length() - 1) {
            if (url.lastIndexOf("%") > i) {
                
                
                
                
            } else {
                port = Integer.parseInt(url.substring(i + 1));
                url = url.substring(0, i);
            }
        }
        if (url.length() > 0) {
            host = url;
        }
        return new URL(protocol, username, password, host, port, path, parameters);
    }
    public static URL valueOf(String url, String... reserveParams) {
        URL result = valueOf(url);
        if (reserveParams == null || reserveParams.length == 0) {
            return result;
        }
        Map<String, String> newMap = new HashMap<>(reserveParams.length);
        Map<String, String> oldMap = result.getParameters();
        for (String reserveParam : reserveParams) {
            String tmp = oldMap.get(reserveParam);
            if (StringUtils.isNotEmpty(tmp)) {
                newMap.put(reserveParam, tmp);
            }
        }
        return result.clearParameters().addParameters(newMap);