Webservice occasionally breaks lp.bugs.subscribers.what_changed

What happens:
IObjectModifiedEvent is a little ambiguous. The definition of its edited_fields attribute is:

    edited_fields = Attribute(
        "The list of fields that were edited. A field name may appear in "
        "this list if it were shown on an edit form, but not actually "
        "changed.")

This doesn't clarify whether the list should be a list of field instances or of field names (though it implies the latter).

In lp.bugs.subscribers.bugactivity.what_changed, edited_fields is expected to be a list of field names (which makes sense in the context of dealing with web forms). However, the WebService appears, under certain circumstances, to create ObjectModifiedEvents where the edited_fields list is a list of field instances rather than fieldnames. This causes what_changed to break in an hilarious fashion.

To reproduce:
Remove the "if IField.providedBy(fieldname):..." block on on line 80 of lib/lp/bugs/subscribers/bugactivity.py and run `bin/test -cvvt webservice/xx-bugs.txt` to exercise the webservice code that causes the problem.