Semantic of Js_of_ocaml.Js.Unsafe.{get, set, delete} is changing
See https://github.com/ocsigen/js_of_ocaml/pull/997
The change will affect Js_of_ocaml.Js.Unsafe.{get, set, delete}
called with ocaml string keys that are not ascii.
A trivial fix is to call them with javascript string instead.
Js.Unsage.get o (Js.string k)
instead of Js.Unsafe.get o k