Fix invalid extend test: _.keys uses hasOwnProp

Fix test to check for the change made in #1770. Currently this test would pass both before and after that change since `_.keys` uses `hasOwnProperty` anyway.
Really it wouldn't matter because `_.extend` will assign the properties as own props. The test was to see if `_.extend` assigned the inherited props of source. The rewrite has less scaffolding so that's good.