This revision corrects a regression introduced by
8395f0d217e16df18fcc6f4309fe74b0dd8d50a2:
the possibility to call
`ir.qweb.field.datetime`.`value_to_html` with
a datetime string directly, instead of calling
`ir.qweb.field.datetime`.`record_to_html`
(which call `value_to_html` as well).
opw-675427