fix changeset calculation with translated documents

it seems that translated documents are always considered changed by the changeset calculation. i guess on phpcr level this is caught as no node actually changes, so this does not lead to massive write queries.

however, when listening to events, this can create severe problems. in my case, i have a list of 80 items to chose from in an admin. those are all loaded, thus all considered changed and instead of 1 event for my changed page, i get 81 events and have no indication that only 1 really did change.
the locale field is not persisted, so changes in this field are irrelevant to determining if to persist only in so much as it changes the locale that is bound. but we are using the fields array as a mean to know which fields we have to examine for changes.