change logic of timestamp "update_time"
change logic of timestamp "update_time"

the old logic seems a hot-fixed of a bug----by the way, the version on npm 0.13.3 was not up-to-date and will throw error when timestamps:['create_at',null]. so I have to use npm github ----I suggest to rewrite it this way. 
Additional discussion: 
1) why check "this.hasChanged(createdAtKey)" when inserting? I don't quite understand. 
2) if update-ts should be set to now when row is inserted? I think setting it to null on creation will provide more information----so people know this records has never been changed. But of course, this is more about convention.
bookshelf/bookshelf,tgriesser/bookshelf,tgriesser/bookshelf,bookshelf/bookshelf