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.

if (value <= 0) {
            return defaultValue;
        }
        return value;
    }
    public char getMethodParameter(String method, String key, char defaultValue) {
        String value = getMethodParameter(method, key);
        if (StringUtils.isEmpty(value)) {
            return defaultValue;
        }
        return value.charAt(0);
    }
    public boolean getMethodParameter(String method, String key, boolean defaultValue) {
        String value = getMethodParameter(method, key);
        if (StringUtils.isEmpty(value)) {
            return defaultValue;
        }
        return Boolean.parseBoolean(value);
    }
    public boolean hasMethodParameter(String method, String key) {
        if (method == null) {
            String suffix = "." + key;
            for (String fullKey : parameters.keySet()) {
                if (fullKey.endsWith(suffix)) {
                    return true;
                }
            }
            return false;
        }
        if (key == null) {