UpDuration = function (key, duration)
{
    if (duration === undefined) { duration = 50; }

    var current = key.plugin.game.loop.time - key.timeUp;

    return (key.isUp && current < duration);
}
Returns `true` if the Key was released within the `duration` value given, based on the current
game clock time. Or returns `false` if it either isn't up, or was released longer ago than the given duration.

@function Phaser.Input.Keyboard.UpDuration
@since 3.0.0

@param {Phaser.Input.Keyboard.Key} key - The Key object to test.
@param {number} [duration=50] - The duration, in ms, within which the key must have been released.

@return {boolean} `true` if the Key was released within `duration` ms ago, otherwise `false`.