Ensure compliancy of Optional serialization for 2.6.0, wrt null checks

[cowtowncoder]

As per discussion on Jackson-dev list, serialization inclusion checks for various "optional" types should work such that 'absent' value is NOT considered null-equivalent (only actual Java null should qualify), but need to be considered "empty" (and "default" where applicable, usually absent would be included but it depends).

This means a change for Guava module to get it in line with JDK8 Optional, and other similar referential types.

Since this is backwards incompatible change, an additional configuration property should be added in GuavaModule. Whether it should default to existing 2.5 behavior or not is open to discussion -- at this point I assume it should.

[cowtowncoder]

Added GuavaModule.configureAbsentsAsNulls(boolean) that allows changing behavior.
Default state retained as true (that is: yes, filter out like nulls, as needed) for backwards compatibility.