fix difference_update
fix difference_update

01-/dedupe,pombredanne/dedupe,tfmorris/dedupe,pombredanne/dedupe,davidkunio/dedupe,nmiranda/dedupe,01-/dedupe,datamade/dedupe,dedupeio/dedupe,dedupeio/dedupe,neozhangthe1/dedupe,neozhangthe1/dedupe,nmiranda/dedupe,tfmorris/dedupe,dedupeio/dedupe-examples,davidkunio/dedupe,datamade/dedupe