kinda clone attrs obj argument

Address #47 

Ideally el.attrs would be an object with no prototype (```Object.create(null)```) so we wouldn't need hasOwnProperty and iterating would be faster but that would break on IE<9.

Also it's not a deep clone but we do not expect the object to be nested anyway.

So it's not perfect but I think it's better that just referencing the passed object. We can improve later.