Fix findbugs warning about wrong comparison via equals in PropertySourceChangeBuilder

PropertySourceChangeBuilder.java:102, EC_UNRELATED_TYPES, Priority: High

Call to org.apache.tamaya.spi.PropertyValue.equals(String) in org.apache.tamaya.events.PropertySourceChangeBuilder.compare(PropertySource, PropertySource)

This method calls equals(Object) on two references of different class types and analysis suggests they will be to objects of different classes at runtime. Further, examination of the equals methods that would be invoked suggest that either this call will always return false, or else the equals method is not be symmetric (which is a property required by the contract for equals in class Object).