Fix of resource loading for application container environments, like spring-boot

[zhemaituk]

closes #99

[Petersoj]

Looks good, however, according to the Javadoc here for class.getClassLoader:

Some implementations may use null to represent the bootstrap class loader. This method will return null in such implementations if this class was loaded by the bootstrap class loader.

So in the event of such a bootstrap class loader, this would fail. Perhaps the following is better:

ClassLoader classLoader = PropertyUtil.class.getClassLoader();
if (classLoader == null) {
    classLoader = ClassLoader.getSystemClassLoader();
}

Thoughts?

[zhemaituk]

Good point. Updated.