yams #109207 [schema] rproperties: stop duck typing [validation pending]

we keep the bw compat for date/datetime/etc. to let people adjust their schemas

for other types, this should be transparent (it was cubicweb that converted back and forth default values to unicode strings)

done in0.39.0
load left0.000
closed by#6b7e541193f6 [schema] cleanup default values handling in entity schema (closes #109207)
patch[schema] default values: cease duck typing practice (except for date/datetimes/... for bw compat) (closes #109207) [applied]