Update CalendarSerializer.java

// _customFormat cannot parse Calendar, so Date should be passed
Improve unit testing for trees vs polymorphic
Update notes wrt #353
Clean up test for #47; have a look at how implement, realizing that it's... a pain to implement.
bit more testing for JsonPointer
merge test to master
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Merge pull request #423 from sergeymetallic/patch-1

Update CalendarSerializer.java
Merge branch 'master' of github.com:FasterXML/jackson-databind
Fixed #390; trying to trim forward-ref implementation slightly to remove non-static inner classes
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refactoring handling of forward-references
refactoring
yet more refactoring, hopefully done for now
Fixed #369
Work on #422
Implement #335
unit test refactoring
Implement #375
Minor code cleanup; should produce better error message for #354