Fixed incorrect result of toJSON for dates with timezone other than UTC

The `toJSON` function makes a conversion that loses timezone information. The result is a string that describes a different moment in time. For example:

    moment("2012-10-09T20:30:40.678+0300").toJSON()
    "2012-10-09T19:30:40.678Z" // depends on your locale setting

but the expected value is

    "2012-10-09T17:30:40.678Z"

The attached patch fixes this problem.