fix: number as key

[anavalo]

Closes: #14

Fixes this behavior: dotize.backward({"2": "2"}) => [ null, null, "2" ] // expect { "2" : "2"}

[vardars]

Hi, thanks for your effort, I really appreciate that. But can you create a new pull request with just what you planned to add/change? In this pr you seem like changed all lines, because of reformatting. Thanks in advance.

[vardars]

Oh and also can you add a test case this pr is solving.