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.

public boolean getMethodBooleanParameter(String method, String key) {
        return getMethodParameter(method, key, false);
    }
    /**
     * @see #getMethodParameter(String, String, boolean)
     * @deprecated Replace to <code>getMethodParameter(String, String, boolean)</code>
     */
    @Deprecated
    public boolean getMethodBooleanParameter(String method, String key, boolean defaultValue) {
        return getMethodParameter(method, key, defaultValue);
    }
    public Configuration toConfiguration() {
        InmemoryConfiguration configuration = new InmemoryConfiguration();
        configuration.addProperties(parameters);
        return configuration;
    }
    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((host == null) ? 0 : host.hashCode());
        result = prime * result + ((parameters == null) ? 0 : parameters.hashCode());
        result = prime * result + ((password == null) ? 0 : password.hashCode());
        result = prime * result + ((path == null) ? 0 : path.hashCode());
        result = prime * result + port;
        result = prime * result + ((protocol == null) ? 0 : protocol.hashCode());
        result = prime * result + ((username == null) ? 0 : username.hashCode());
        return result;
    }
    @Override