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`__datetime__` gets changed into `datetime`.
See https://github.com/plomino/Plomino/issues/453
Attempt to document and debug

- Let values on `request` trump (default) computed value if editable.
- Don't ignore `request` parameter (i.e. don't use
  `self.context.REQUEST`).
The value of a date field should already be a date

If it needs to be coerced to a date, that needs to happen when setting
the value, not when reading it.
DateTime JSON serialization changed
Exciting, the order of the dict changes -_-
Merge pull request #454 from plomino/jean-BaseField-getFieldValue-default

`BaseField.getFieldValue` default handling