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 value;
    }
    public char getParameter(String key, char defaultValue) {
        String value = getParameter(key);
        if (StringUtils.isEmpty(value)) {
            return defaultValue;
        }
        return value.charAt(0);
    }
    public boolean getParameter(String key, boolean defaultValue) {
        String value = getParameter(key);
        if (StringUtils.isEmpty(value)) {
            return defaultValue;
        }
        return Boolean.parseBoolean(value);
    }
    public boolean hasParameter(String key) {
        String value = getParameter(key);
        return value != null && value.length() > 0;
    }
    public String getMethodParameterAndDecoded(String method, String key) {
        return URL.decode(getMethodParameter(method, key));
    }
    public String getMethodParameterAndDecoded(String method, String key, String defaultValue) {
        return URL.decode(getMethodParameter(method, key, defaultValue));
    }
    public String getMethodParameter(String method, String key) {
        String value = parameters.get(method + "." + key);
        if (StringUtils.isEmpty(value)) {