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 int getPort() {
        return port;
    }
    public URL setPort(int port) {
        return new URL(protocol, username, password, host, port, path, getParameters());
    }
    public int getPort(int defaultPort) {
        return port <= 0 ? defaultPort : port;
    }
    public String getAddress() {
        return port <= 0 ? host : host + ":" + port;
    }
    public URL setAddress(String address) {
        int i = address.lastIndexOf(':');
        String host;
        int port = this.port;
        if (i >= 0) {
            host = address.substring(0, i);
            port = Integer.parseInt(address.substring(i + 1));
        } else {
            host = address;
        }
        return new URL(protocol, username, password, host, port, path, getParameters());
    }
    public String getBackupAddress() {
        return getBackupAddress(0);
    }
    public String getBackupAddress(int defaultPort) {
        StringBuilder address = new StringBuilder(appendDefaultPort(getAddress(), defaultPort));
        String[] backups = getParameter(Constants.BACKUP_KEY, new String[0]);