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 URL removeParameter(String key) {
        if (StringUtils.isEmpty(key)) {
            return this;
        }
        return removeParameters(key);
    }
    public URL removeParameters(Collection<String> keys) {
        if (CollectionUtils.isEmpty(keys)) {
            return this;
        }
        return removeParameters(keys.toArray(new String[0]));
    }
    public URL removeParameters(String... keys) {
        if (keys == null || keys.length == 0) {
            return this;
        }
        Map<String, String> map = new HashMap<>(getParameters());
        for (String key : keys) {
            map.remove(key);
        }
        if (map.size() == getParameters().size()) {
            return this;
        }
        return new URL(protocol, username, password, host, port, path, map);
    }
    public URL clearParameters() {
        return new URL(protocol, username, password, host, port, path, new HashMap<>());
    }
    public String getRawParameter(String key) {