resultset.set ignores slice, modifying data it shouldn't

[Stuart Bishop]

This is a potential data loss bug.

For a result set foo, foo[:n].set(...) updated all rows selected by the foo result set, not just the sliced rows.

>>> bugs = store.find(Bug)
>>> limited_bugs = bugs[:3]
>>> list(limited_bugs)
SELECT Bug.date_last_message, Bug.date_last_updated, Bug.date_made_private, Bug.datecreated, Bug.description, Bug.duplicateof, Bug.heat, Bug.heat_last_updated, Bug.id, Bug.latest_patch_uploaded, Bug.message_count, Bug.name, Bug.number_of_duplicates, Bug.owner, Bug.private, Bug.security_related, Bug.title, Bug.users_affected_count, Bug.users_unaffected_count, Bug.who_made_private FROM Bug ORDER BY Bug.id DESC LIMIT 3
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[<Bug at 0x791fb90>, <Bug at 0x791fa10>, <Bug at 0x791f890>]
>>> limited_bugs.set(heat_last_updated=None)
UPDATE Bug SET heat_last_updated=NULL
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