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 addParameter(key, String.valueOf(value));
    }
    public URL addParameter(String key, short value) {
        return addParameter(key, String.valueOf(value));
    }
    public URL addParameter(String key, int value) {
        return addParameter(key, String.valueOf(value));
    }
    public URL addParameter(String key, long value) {
        return addParameter(key, String.valueOf(value));
    }
    public URL addParameter(String key, float value) {
        return addParameter(key, String.valueOf(value));
    }
    public URL addParameter(String key, double value) {
        return addParameter(key, String.valueOf(value));
    }
    public URL addParameter(String key, Enum<?> value) {
        if (value == null) {
            return this;
        }
        return addParameter(key, String.valueOf(value));
    }
    public URL addParameter(String key, Number value) {
        if (value == null) {
            return this;
        }
        return addParameter(key, String.valueOf(value));
    }
    public URL addParameter(String key, CharSequence value) {