Fix error in Map.difference test

Author: mschristiansen

test/Test/Data/Map.purs

@@ -218,7 +218,7 @@ mapTests = do
   log "difference"
   quickCheck $ \(TestMap m1) (TestMap m2) ->
     let d = M.difference m1 m2 :: M.Map SmallKey Int
-    in and (map (\k -> M.member k d) (A.fromFoldable $ M.keys d)) &&
+    in and (map (\k -> M.member k m1) (A.fromFoldable $ M.keys d)) &&
        and (map (\k -> not $ M.member k d) (A.fromFoldable $ M.keys m2))
 
   log "size"