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.

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        String value = getMethodParameter(method, key);
        if (StringUtils.isEmpty(value)) {
            return defaultValue;
        }
        float f = Float.parseFloat(value);
        getNumbers().put(methodKey, f);
        return f;
    }
    public long getMethodParameter(String method, String key, long defaultValue) {
        String methodKey = method + "." + key;
        Number n = getNumbers().get(methodKey);
        if (n != null) {
            return n.longValue();
        }
        String value = getMethodParameter(method, key);
        if (StringUtils.isEmpty(value)) {
            return defaultValue;
        }
        long l = Long.parseLong(value);
        getNumbers().put(methodKey, l);
        return l;
    }
    public int getMethodParameter(String method, String key, int defaultValue) {
        String methodKey = method + "." + key;
        Number n = getNumbers().get(methodKey);
        if (n != null) {
            return n.intValue();
        }
        String value = getMethodParameter(method, key);