jsx: className attr transform is applied to "uppercase components"

It seems JSX ppx renames className attribute to class for elements generated from uppercase components. This results in the createElement application being passed a ~class argument, which doesn't make sense as it's a reserved word.

To repro:

module Foo = {
  let createElement = (~className, ()) => {
    <div className />;
  };
};
let el = <Foo className="bar" />

Error:

The function applied to this argument has type
  (~className: Tyxml.Html.wrap(Html_types.nmtokens)) => Tyxml_html.elt([> Html_types.div ])
This argument cannot be applied with label ~class

Should attribute mangling only be applied to elements generated from lowercase components?

Start by locating the JSX ppx transformation that handles className attributes and compare its behavior for lowercase elements versus uppercase components. Reproduce the example from the issue, then verify that the generated createElement call preserves the ~className label for the uppercase component case.