Add date en/decoding to JSON.

This is an experiment. Are we sure that the patching in PlominoUtils will
always happen before we use json?
Merged github-main
Local changes committed during merge.
Keep StringToDate backwards compatible.
Patch jsonutils from __init__ instead of PlominoUtils.
Forgot import.
Don't need name mangling.
fix DateTime/datetime conversion
Merge pull request #266 from plomino/jean-263-json-dates

Add date en/decoding to JSON.