### Environment

* Dubbo version: 2.7.3
* Operating System version: CentOS7
* Java version: 1.8

### Steps to reproduce this issue

1. Set two protocal, with different "register" attribute

```xml
<dubbo:protocol name="dubbo" port="8751" register="true"  />
<dubbo:protocol name="hessian" port="8752" register="false" />
```

2. use the zookeeper registry center

```xml
<dubbo:registry address="..." file="./.dubbo_cache" />
```

3. start the server and view the provider in zkCli


### Expected Result
only the dubbo is registed

### Actual Result
the dubbo and hession protocol are registed

@Override
    public Boolean getExport() {
        return (export == null && provider != null) ? provider.getExport() : export;
    }
    private boolean shouldDelay() {
        Integer delay = getDelay();
        return delay != null && delay > 0;
    }
    @Override
    public Integer getDelay() {
        return (delay == null && provider != null) ? provider.getDelay() : delay;
    }
    protected synchronized void doExport() {
        if (unexported) {
            throw new IllegalStateException("The service " + interfaceClass.getName() + " has already unexported!");
        }
        if (exported) {
            return;
        }
        exported = true;
        if (StringUtils.isEmpty(path)) {
            path = interfaceName;
        }
        doExportUrls();
    }
    private void checkRef() {
        // 
        if (ref == null) {
            throw new IllegalStateException("ref not allow null!");
        }