I'm curious about the method:getSubProperties

### Environment

* Dubbo version: 2.7.5-SNAPSHOT
* Operating System version: win10
* Java version: 1.8.0_111

### Steps to reproduce this issue

1. 为什么解析@EnableDubboConfigBinding的每一行配置都要从springEnv引用到 new AbstractEnvironment()
见org/apache/dubbo/config/spring/util/PropertySourcesUtils.java:52
如下：
public static Map getSubProperties(Iterable> propertySources, String prefix) {

// Non-Extension AbstractEnvironment
AbstractEnvironment environment = new AbstractEnvironment() {
};

MutablePropertySources mutablePropertySources = environment.getPropertySources();

for (PropertySource source : propertySources) {
mutablePropertySources.addLast(source);
}