myBin.get() returns undefined and removes localStorage item

If I create a new Bin with no expiry and set it, getting it returns `undefined` and immediately removes the `localStorage` item.

https://codepen.io/blakewatson/pen/GRQLZjr?editors=1012

I think the problem is here: https://github.com/cferdinandi/bin/blob/master/src/bin.js#L51

`this.expires` is `undefined`, which returns `false` from `isValid`'s check for `null`. I’m thinking that line was supposed to use `data.expires` so that the value would be `null` and thus `isValid` would return `true`.

Start at src/bin.js line 51 and reproduce the no-expiry case using the linked CodePen. Trace how an undefined expiry is handled by isValid, then verify that creating, setting, and getting a Bin without expiry returns the value and preserves its localStorage item.

Written by the indexing model from the issue text.