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 String toFullString() {
        if (full != null) {
            return full;
        }
        return full = buildString(true, true);
    }
    public String toFullString(String... parameters) {
        return buildString(true, true, parameters);
    }
    public String toParameterString() {
        if (parameter != null) {
            return parameter;
        }
        return parameter = toParameterString(new String[0]);
    }
    public String toParameterString(String... parameters) {
        StringBuilder buf = new StringBuilder();
        buildParameters(buf, false, parameters);
        return buf.toString();
    }
    private void buildParameters(StringBuilder buf, boolean concat, String[] parameters) {
        if (CollectionUtils.isNotEmptyMap(getParameters())) {
            List<String> includes = (ArrayUtils.isEmpty(parameters) ? null : Arrays.asList(parameters));
            boolean first = true;
            for (Map.Entry<String, String> entry : new TreeMap<>(getParameters()).entrySet()) {
                if (entry.getKey() != null && entry.getKey().length() > 0
                        && (includes == null || includes.contains(entry.getKey()))) {
                    if (first) {
                        if (concat) {