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.

return new URL(protocol, username, password, host, port, path, getParameters());
    }
    public String getAuthority() {
        if (StringUtils.isEmpty(username)
                && StringUtils.isEmpty(password)) {
            return null;
        }
        return (username == null ? "" : username)
                + ":" + (password == null ? "" : password);
    }
    public String getHost() {
        return host;
    }
    public URL setHost(String host) {
        return new URL(protocol, username, password, host, port, path, getParameters());
    }
    /**
     * Fetch IP address for this URL.
     * <p>
     * Pls. note that IP should be used instead of Host when to compare with socket's address or to search in a map
     * which use address as its key.
     *
     * @return ip in string format
     */
    public String getIp() {
        if (ip == null) {
            ip = NetUtils.getIpByHost(host);
        }
        return ip;
    }