[Fix] Eliom_client.rebuild_{r,}attrib

It seams previous fixes were wrong.
setAttribute works as expected.
The issue was coming from the use
of `Js.Unsafe.set node name v` for int
and float attributes.

That said,
`elt.setAttribute('value', val)` no longer works after value property
has be set (`elt.value = otherval`).

https://developer.mozilla.org/en-US/docs/Web/API/Element.setAttribute
Using setAttribute() to modify certain attributes, most notably value
in XUL, works inconsistently, as the attribute specifies the default
value. To access or modify the current values, you should use the
properties. For example, use elt.value instead of
elt.setAttribute('value', val).