Creating a moment with a timezone from another moment.

Currently, this is happening.

```js
var a = moment.tz([2014, 9, 1], 'Europe/London');
var b = moment.tz(a, 'Europe/London');
console.log(a.format()); // 2014-10-01T00:00:00+01:00
console.log(b.format()); // 2014-09-30T23:00:00+01:00
```

Instead, this should happen.

```js
var a = moment.tz([2014, 9, 1], 'Europe/London');
var b = moment.tz(a, 'Europe/London');
console.log(a.format()); // 2014-10-01T00:00:00+01:00
console.log(b.format()); // 2014-10-01T00:00:00+01:00
```