XmlHttpRequest: fix bug with ie: responseXML is not null when the content is not xml, but documentElement is.
XmlHttpRequest: fix bug with ie: responseXML is not null when the content is not xml, but documentElement is.

Ignore-this: 398602370c6c57f15a0a53afd6d0dcf2

darcs-hash:20111216145911-0cb73-0e4c7311f4e9cff5012e035a8053f951856d342c

alainfrisch/js_of_ocaml,alainfrisch/js_of_ocaml,alainfrisch/js_of_ocaml,alainfrisch/js_of_ocaml,OCamlPro-Henry/js_of_ocaml,OCamlPro-Henry/js_of_ocaml,OCamlPro-Henry/js_of_ocaml,alainfrisch/js_of_ocaml,OCamlPro-Henry/js_of_ocaml,OCamlPro-Henry/js_of_ocaml