Fix durations set to 0 being ignored with Velocity UI

[jackwakefield]

This fixes the following where the duration would be set to the effects default.

$element.velocity('transition.swoopIn', { duration: 0 });

[Rycochet]

Just a general observation - but you should be using if (var !== undefined) rather than using typeof - while there are (mostly irrelevant) arguments about it's use, it's the standard, and used in Velocity source already - so you should try to be consistent when contributing to an OS project :-)