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.

} catch (UnsupportedEncodingException e) {
            throw new RuntimeException(e.getMessage(), e);
        }
    }
    public static String decode(String value) {
        if (StringUtils.isEmpty(value)) {
            return "";
        }
        try {
            return URLDecoder.decode(value, "UTF-8");
        } catch (UnsupportedEncodingException e) {
            throw new RuntimeException(e.getMessage(), e);
        }
    }
    public String getProtocol() {
        return protocol;
    }
    public URL setProtocol(String protocol) {
        return new URL(protocol, username, password, host, port, path, getParameters());
    }
    public String getUsername() {
        return username;
    }
    public URL setUsername(String username) {
        return new URL(protocol, username, password, host, port, path, getParameters());
    }
    public String getPassword() {
        return password;
    }
    public URL setPassword(String password) {