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.

assertTrue(url.isLocalHost());
    }
    @Test
    public void test_Path() throws Exception {
        URL url = new URL("dubbo", "localhost", 20880, "////path");
        assertEquals("path", url.getPath());
    }
    @Test
    public void testAddParameters() throws Exception {
        URL url = URL.valueOf("dubbo://127.0.0.1:20880");
        Map<String, String> parameters = new HashMap<String, String>();
        parameters.put("version", null);
        url.addParameters(parameters);
    }
    @Test
    public void testUserNamePasswordContainsAt(){
        
        URL url = URL.valueOf("ad@min:hello@1234@10.20.130.230:20880/context/path?version=1.0.0&application=morgan");
        assertNull(url.getProtocol());
        assertEquals("ad@min", url.getUsername());
        assertEquals("hello@1234", url.getPassword());
        assertEquals("10.20.130.230", url.getHost());
        assertEquals(20880, url.getPort());
        assertEquals("context/path", url.getPath());
        assertEquals(2, url.getParameters().size());
        assertEquals("1.0.0", url.getParameter("version"));
        assertEquals("morgan", url.getParameter("application"));
    }
    @Test
    public void testIpV6Address(){