Feature: Default setting for mapping of null values wanted

[schuettec]

The mapper currently does not map null values from source to destination. In a context of REST operations like PUT, this behaviour is not the best choice as default mapping behaviour.

Since version 4.2.6 the mapper can be configured to map null values: MappingConfiguration.writeNullIfSourceIsNull().

This behaviour should be implemented as configurable default with the possibility to configure all mappers in a global way.