Droppable removes all class names on hover when activeclass not initialized

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The ​Interface Droppable documentation lists activeclass as an optional parameter. However, when no value is provided, hovering causes all class names to be removed from the element.

Furthermore, if the value is omitted, rather than being initialized to an empty string, "undefined" is appended to the element's class in FF 1.5.

This will probably require changes at least to lines 140 and 160 in idrop.js. Possibly 62, 102, 181 and 227, as well.