cubicweb #3712885 Dataimport and dates [validation pending]

We shouls serialize date and datetime in two different fashions in dataimport _create_copyfrom_buffer.

prioritynormal
typebug
done in3.18.5
load0.100
load left0.000
closed by#4de6e73f580d [dataimport] Correctly serialize datetime objects in _create_copyfrom_buffer, closes #3712885
patch[dataimport] Correctly serialize datetime objects in _create_copyfrom_buffer, closes #3712885 [applied]