default. 导致的高版本与低版本间的服务发现问题（default.version default.group）

lower version, for example 2.6.8
```properties
dubbo.provider.version=1.0
```

2.7.6 failed to recognize the provider url with 'default.version=1.0' key,  it only takes 'version.1.0' into consideration.
