Eliom_client: fix typing error

The code was still correct, Js engine calling 'toString()' to get a js string

See fix in jsoo
https://github.com/ocsigen/js_of_ocaml/commit/33ba7c3a4af4412acac5700263288c3612ed44df
Fix previous commit
Merge remote-tracking branch 'upstream/master' into ocsigenserver-3.0.0