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.

/**
     * Add parameters to a new url.
     *
     * @param parameters parameters in key-value pairs
     * @return A new URL
     */
    public URL addParameters(Map<String, String> parameters) {
        if (CollectionUtils.isEmptyMap(parameters)) {
            return this;
        }
        boolean hasAndEqual = true;
        for (Map.Entry<String, String> entry : parameters.entrySet()) {
            String value = getParameters().get(entry.getKey());
            if (value == null) {
                if (entry.getValue() != null) {
                    hasAndEqual = false;
                    break;
                }
            } else {
                if (!value.equals(entry.getValue())) {
                    hasAndEqual = false;
                    break;
                }
            }
        }
        
        if (hasAndEqual) {
            return this;
        }
        Map<String, String> map = new HashMap<>(getParameters());